please can you paste the contents of autoload_psr4.php

/<?php defined('_JEXEC') or die; return [ 'Joomla\\Component\\Actionlogs\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_actionlogs/src'], 'Joomla\\Component\\Admin\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_admin/src'], 'Joomla\\Component\\Associations\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_associations/src'], 'Joomla\\Component\\Banners\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_banners/src'], 'Joomla\\Component\\Banners\\Api\\' => [JPATH_API . '/components/com_banners/src'], 'Joomla\\Component\\Banners\\Site\\' => [JPATH_SITE . '/components/com_banners/src'], 'Joomla\\Component\\Cache\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_cache/src'], 'Joomla\\Component\\Categories\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_categories/src'], 'Joomla\\Component\\Categories\\Api\\' => [JPATH_API . '/components/com_categories/src'], 'Joomla\\Component\\Checkin\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_checkin/src'], 'Joomla\\Component\\Config\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_config/src'], 'Joomla\\Component\\Config\\Api\\' => [JPATH_API . '/components/com_config/src'], 'Joomla\\Component\\Config\\Site\\' => [JPATH_SITE . '/components/com_config/src'], 'Joomla\\Component\\Contact\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_contact/src'], 'Joomla\\Component\\Contact\\Api\\' => [JPATH_API . '/components/com_contact/src'], 'Joomla\\Component\\Contact\\Site\\' => [JPATH_SITE . '/components/com_contact/src'], 'Joomla\\Component\\Content\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_content/src'], 'Joomla\\Component\\Content\\Api\\' => [JPATH_API . '/components/com_content/src'], 'Joomla\\Component\\Content\\Site\\' => [JPATH_SITE . '/components/com_content/src'], 'Joomla\\Component\\Contenthistory\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_contenthistory/src'], 'Joomla\\Component\\Contenthistory\\Api\\' => [JPATH_API . '/components/com_contenthistory/src'], 'Joomla\\Component\\Contenthistory\\Site\\' => [JPATH_SITE . '/components/com_contenthistory/src'], 'Joomla\\Component\\Cpanel\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_cpanel/src'], 'Joomla\\Component\\Fields\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_fields/src'], 'Joomla\\Component\\Fields\\Api\\' => [JPATH_API . '/components/com_fields/src'], 'Joomla\\Component\\Fields\\Site\\' => [JPATH_SITE . '/components/com_fields/src'], 'Joomla\\Component\\Finder\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_finder/src'], 'Joomla\\Component\\Finder\\Site\\' => [JPATH_SITE . '/components/com_finder/src'], 'Joomla\\Component\\Guidedtours\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_guidedtours/src'], 'Joomla\\Component\\Installer\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_installer/src'], 'Joomla\\Component\\Installer\\Api\\' => [JPATH_API . '/components/com_installer/src'], 'Joomla\\Component\\Joomlaupdate\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_joomlaupdate/src'], 'Joomla\\Component\\Languages\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_languages/src'], 'Joomla\\Component\\Languages\\Api\\' => [JPATH_API . '/components/com_languages/src'], 'Joomla\\Component\\Login\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_login/src'], 'Joomla\\Component\\Mails\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_mails/src'], 'Joomla\\Component\\Media\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_media/src'], 'Joomla\\Component\\Media\\Api\\' => [JPATH_API . '/components/com_media/src'], 'Joomla\\Component\\Media\\Site\\' => [JPATH_SITE . '/components/com_media/src'], 'Joomla\\Component\\Menus\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_menus/src'], 'Joomla\\Component\\Menus\\Api\\' => [JPATH_API . '/components/com_menus/src'], 'Joomla\\Component\\Menus\\Site\\' => [JPATH_SITE . '/components/com_menus/src'], 'Joomla\\Component\\Messages\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_messages/src'], 'Joomla\\Component\\Messages\\Api\\' => [JPATH_API . '/components/com_messages/src'], 'Joomla\\Component\\Modules\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_modules/src'], 'Joomla\\Component\\Modules\\Api\\' => [JPATH_API . '/components/com_modules/src'], 'Joomla\\Component\\Modules\\Site\\' => [JPATH_SITE . '/components/com_modules/src'], 'Joomla\\Component\\Newsfeeds\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_newsfeeds/src'], 'Joomla\\Component\\Newsfeeds\\Api\\' => [JPATH_API . '/components/com_newsfeeds/src'], 'Joomla\\Component\\Newsfeeds\\Site\\' => [JPATH_SITE . '/components/com_newsfeeds/src'], 'Joomla\\Component\\Plugins\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_plugins/src'], 'Joomla\\Component\\Plugins\\Api\\' => [JPATH_API . '/components/com_plugins/src'], 'Joomla\\Component\\Postinstall\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_postinstall/src'], 'Joomla\\Component\\Privacy\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_privacy/src'], 'Joomla\\Component\\Privacy\\Api\\' => [JPATH_API . '/components/com_privacy/src'], 'Joomla\\Component\\Privacy\\Site\\' => [JPATH_SITE . '/components/com_privacy/src'], 'Joomla\\Component\\Redirect\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_redirect/src'], 'Joomla\\Component\\Redirect\\Api\\' => [JPATH_API . '/components/com_redirect/src'], 'Joomla\\Component\\Scheduler\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_scheduler/src'], 'Joomla\\Component\\Tags\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_tags/src'], 'Joomla\\Component\\Tags\\Api\\' => [JPATH_API . '/components/com_tags/src'], 'Joomla\\Component\\Tags\\Site\\' => [JPATH_SITE . '/components/com_tags/src'], 'Joomla\\Component\\Templates\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_templates/src'], 'Joomla\\Component\\Templates\\Api\\' => [JPATH_API . '/components/com_templates/src'], 'Joomla\\Component\\Users\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_users/src'], 'Joomla\\Component\\Users\\Api\\' => [JPATH_API . '/components/com_users/src'], 'Joomla\\Component\\Users\\Site\\' => [JPATH_SITE . '/components/com_users/src'], 'Joomla\\Component\\Workflow\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_workflow/src'], 'Joomla\\Component\\Wrapper\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_wrapper/src'], 'Joomla\\Component\\Wrapper\\Site\\' => [JPATH_SITE . '/components/com_wrapper/src'], 'Joomla\\Module\\ArticlesArchive\\Site\\' => [JPATH_SITE . '/modules/mod_articles_archive/src'], 'Joomla\\Module\\ArticlesCategories\\Site\\' => [JPATH_SITE . '/modules/mod_articles_categories/src'], 'Joomla\\Module\\ArticlesCategory\\Site\\' => [JPATH_SITE . '/modules/mod_articles_category/src'], 'Joomla\\Module\\ArticlesLatest\\Site\\' => [JPATH_SITE . '/modules/mod_articles_latest/src'], 'Joomla\\Module\\ArticlesNews\\Site\\' => [JPATH_SITE . '/modules/mod_articles_news/src'], 'Joomla\\Module\\ArticlesPopular\\Site\\' => [JPATH_SITE . '/modules/mod_articles_popular/src'], 'Joomla\\Module\\Banners\\Site\\' => [JPATH_SITE . '/modules/mod_banners/src'], 'Joomla\\Module\\Breadcrumbs\\Site\\' => [JPATH_SITE . '/modules/mod_breadcrumbs/src'], 'Joomla\\Module\\Feed\\Administrator\\' => [JPATH_ADMINISTRATOR . '/modules/mod_feed/src'], 'Joomla\\Module\\Feed\\Site\\' => [JPATH_SITE . '/modules/mod_feed/src'], 'Joomla\\Module\\Finder\\Site\\' => [JPATH_SITE . '/modules/mod_finder/src'], 'Joomla\\Module\\GuidedTours\\Administrator\\' => [JPATH_ADMINISTRATOR . '/modules/mod_guidedtours/src'], 'Joomla\\Module\\Languages\\Site\\' => [JPATH_SITE . '/modules/mod_languages/src'], 'Joomla\\Module\\LatestActions\\Administrator\\' => [JPATH_ADMINISTRATOR . '/modules/mod_latestactions/src'], 'Joomla\\Module\\Latest\\Administrator\\' => [JPATH_ADMINISTRATOR . '/modules/mod_latest/src'], 'Joomla\\Module\\Logged\\Administrator\\' => [JPATH_ADMINISTRATOR . '/modules/mod_logged/src'], 'Joomla\\Module\\Login\\Administrator\\' => [JPATH_ADMINISTRATOR . '/modules/mod_login/src'], 'Joomla\\Module\\Login\\Site\\' => [JPATH_SITE . '/modules/mod_login/src'], 'Joomla\\Module\\Menu\\Administrator\\' => [JPATH_ADMINISTRATOR . '/modules/mod_menu/src'], 'Joomla\\Module\\Menu\\Site\\' => [JPATH_SITE . '/modules/mod_menu/src'], 'Joomla\\Module\\Popular\\Administrator\\' => [JPATH_ADMINISTRATOR . '/modules/mod_popular/src'], 'Joomla\\Module\\PrivacyDashboard\\Administrator\\' => [JPATH_ADMINISTRATOR . '/modules/mod_privacy_dashboard/src'], 'Joomla\\Module\\PrivacyStatus\\Administrator\\' => [JPATH_ADMINISTRATOR . '/modules/mod_privacy_status/src'], 'Joomla\\Module\\Quickicon\\Administrator\\' => [JPATH_ADMINISTRATOR . '/modules/mod_quickicon/src'], 'Joomla\\Module\\RandomImage\\Site\\' => [JPATH_SITE . '/modules/mod_random_image/src'], 'Joomla\\Module\\RelatedItems\\Site\\' => [JPATH_SITE . '/modules/mod_related_items/src'], 'Joomla\\Module\\Sampledata\\Administrator\\' => [JPATH_ADMINISTRATOR . '/modules/mod_sampledata/src'], 'Joomla\\Module\\StatsAdmin\\Administrator\\' => [JPATH_ADMINISTRATOR . '/modules/mod_stats_admin/src'], 'Joomla\\Module\\Stats\\Site\\' => [JPATH_SITE . '/modules/mod_stats/src'], 'Joomla\\Module\\Submenu\\Administrator\\' => [JPATH_ADMINISTRATOR . '/modules/mod_submenu/src'], 'Joomla\\Module\\Syndicate\\Site\\' => [JPATH_SITE . '/modules/mod_syndicate/src'], 'Joomla\\Module\\TagsPopular\\Site\\' => [JPATH_SITE . '/modules/mod_tags_popular/src'], 'Joomla\\Module\\TagsSimilar\\Site\\' => [JPATH_SITE . '/modules/mod_tags_similar/src'], 'Joomla\\Module\\UsersLatest\\Site\\' => [JPATH_SITE . '/modules/mod_users_latest/src'], 'Joomla\\Module\\Version\\Administrator\\' => [JPATH_ADMINISTRATOR . '/modules/mod_version/src'], 'Joomla\\Module\\Whosonline\\Site\\' => [JPATH_SITE . '/modules/mod_whosonline/src'], 'Joomla\\Module\\Wrapper\\Site\\' => [JPATH_SITE . '/modules/mod_wrapper/src'], 'Joomla\\Plugin\\Actionlog\\Joomla\\' => [JPATH_PLUGINS . '/actionlog/joomla/src'], 'Joomla\\Plugin\\ApiAuthentication\\Basic\\' => [JPATH_PLUGINS . '/api-authentication/basic/src'], 'Joomla\\Plugin\\ApiAuthentication\\Token\\' => [JPATH_PLUGINS . '/api-authentication/token/src'], 'Joomla\\Plugin\\Authentication\\Cookie\\' => [JPATH_PLUGINS . '/authentication/cookie/src'], 'Joomla\\Plugin\\Authentication\\Joomla\\' => [JPATH_PLUGINS . '/authentication/joomla/src'], 'Joomla\\Plugin\\Authentication\\Ldap\\' => [JPATH_PLUGINS . '/authentication/ldap/src'], 'Joomla\\Plugin\\Behaviour\\Taggable\\' => [JPATH_PLUGINS . '/behaviour/taggable/src'], 'Joomla\\Plugin\\Behaviour\\Versionable\\' => [JPATH_PLUGINS . '/behaviour/versionable/src'], 'Joomla\\Plugin\\Captcha\\InvisibleReCaptcha\\' => [JPATH_PLUGINS . '/captcha/recaptcha_invisible/src'], 'Joomla\\Plugin\\Captcha\\ReCaptcha\\' => [JPATH_PLUGINS . '/captcha/recaptcha/src'], 'Joomla\\Plugin\\Content\\ConfirmConsent\\' => [JPATH_PLUGINS . '/content/confirmconsent/src'], 'Joomla\\Plugin\\Content\\Contact\\' => [JPATH_PLUGINS . '/content/contact/src'], 'Joomla\\Plugin\\Content\\EmailCloak\\' => [JPATH_PLUGINS . '/content/emailcloak/src'], 'Joomla\\Plugin\\Content\\Fields\\' => [JPATH_PLUGINS . '/content/fields/src'], 'Joomla\\Plugin\\EditorsXtd\\Article\\' => [JPATH_PLUGINS . '/editors-xtd/article/src'], 'Joomla\\Plugin\\EditorsXtd\\Contact\\' => [JPATH_PLUGINS . '/editors-xtd/contact/src'], 'Joomla\\Plugin\\EditorsXtd\\Fields\\' => [JPATH_PLUGINS . '/editors-xtd/fields/src'], 'Joomla\\Plugin\\EditorsXtd\\Image\\' => [JPATH_PLUGINS . '/editors-xtd/image/src'], 'Joomla\\Plugin\\EditorsXtd\\Menu\\' => [JPATH_PLUGINS . '/editors-xtd/menu/src'], 'Joomla\\Plugin\\EditorsXtd\\Module\\' => [JPATH_PLUGINS . '/editors-xtd/module/src'], 'Joomla\\Plugin\\EditorsXtd\\ReadMore\\' => [JPATH_PLUGINS . '/editors-xtd/readmore/src'], 'Joomla\\Plugin\\Editors\\CodeMirror\\' => [JPATH_PLUGINS . '/editors/codemirror/src'], 'Joomla\\Plugin\\Editors\\None\\' => [JPATH_PLUGINS . '/editors/none/src'], 'Joomla\\Plugin\\Editors\\TinyMCE\\' => [JPATH_PLUGINS . '/editors/tinymce/src'], 'Joomla\\Plugin\\Extension\\Finder\\' => [JPATH_PLUGINS . '/extension/finder/src'], 'Joomla\\Plugin\\Extension\\Joomla\\' => [JPATH_PLUGINS . '/extension/joomla/src'], 'Joomla\\Plugin\\Extension\\NamespaceMap\\' => [JPATH_PLUGINS . '/extension/namespacemap/src'], 'Joomla\\Plugin\\Fields\\Calendar\\' => [JPATH_PLUGINS . '/fields/calendar/src'], 'Joomla\\Plugin\\Fields\\Checkboxes\\' => [JPATH_PLUGINS . '/fields/checkboxes/src'], 'Joomla\\Plugin\\Fields\\Color\\' => [JPATH_PLUGINS . '/fields/color/src'], 'Joomla\\Plugin\\Fields\\Editor\\' => [JPATH_PLUGINS . '/fields/editor/src'], 'Joomla\\Plugin\\Fields\\Imagelist\\' => [JPATH_PLUGINS . '/fields/imagelist/src'], 'Joomla\\Plugin\\Fields\\Integer\\' => [JPATH_PLUGINS . '/fields/integer/src'], 'Joomla\\Plugin\\Fields\\ListField\\' => [JPATH_PLUGINS . '/fields/list/src'], 'Joomla\\Plugin\\Fields\\Media\\' => [JPATH_PLUGINS . '/fields/media/src'], 'Joomla\\Plugin\\Fields\\Radio\\' => [JPATH_PLUGINS . '/fields/radio/src'], 'Joomla\\Plugin\\Fields\\SQL\\' => [JPATH_PLUGINS . '/fields/sql/src'], 'Joomla\\Plugin\\Fields\\Subform\\' => [JPATH_PLUGINS . '/fields/subform/src'], 'Joomla\\Plugin\\Fields\\Text\\' => [JPATH_PLUGINS . '/fields/text/src'], 'Joomla\\Plugin\\Fields\\Textarea\\' => [JPATH_PLUGINS . '/fields/textarea/src'], 'Joomla\\Plugin\\Fields\\Url\\' => [JPATH_PLUGINS . '/fields/url/src'], 'Joomla\\Plugin\\Fields\\User\\' => [JPATH_PLUGINS . '/fields/user/src'], 'Joomla\\Plugin\\Fields\\UsergroupList\\' => [JPATH_PLUGINS . '/fields/usergrouplist/src'], 'Joomla\\Plugin\\Filesystem\\Local\\' => [JPATH_PLUGINS . '/filesystem/local/src'], 'Joomla\\Plugin\\Finder\\Categories\\' => [JPATH_PLUGINS . '/finder/categories/src'], 'Joomla\\Plugin\\Finder\\Contacts\\' => [JPATH_PLUGINS . '/finder/contacts/src'], 'Joomla\\Plugin\\Finder\\Content\\' => [JPATH_PLUGINS . '/finder/content/src'], 'Joomla\\Plugin\\Finder\\Newsfeeds\\' => [JPATH_PLUGINS . '/finder/newsfeeds/src'], 'Joomla\\Plugin\\Finder\\Tags\\' => [JPATH_PLUGINS . '/finder/tags/src'], 'Joomla\\Plugin\\Multifactorauth\\Email\\' => [JPATH_PLUGINS . '/multifactorauth/email/src'], 'Joomla\\Plugin\\Multifactorauth\\Fixed\\' => [JPATH_PLUGINS . '/multifactorauth/fixed/src'], 'Joomla\\Plugin\\Multifactorauth\\Totp\\' => [JPATH_PLUGINS . '/multifactorauth/totp/src'], 'Joomla\\Plugin\\Multifactorauth\\Webauthn\\' => [JPATH_PLUGINS . '/multifactorauth/webauthn/src'], 'Joomla\\Plugin\\Multifactorauth\\Yubikey\\' => [JPATH_PLUGINS . '/multifactorauth/yubikey/src'], 'Joomla\\Plugin\\Quickicon\\Downloadkey\\' => [JPATH_PLUGINS . '/quickicon/downloadkey/src'], 'Joomla\\Plugin\\Quickicon\\Extensionupdate\\' => [JPATH_PLUGINS . '/quickicon/extensionupdate/src'], 'Joomla\\Plugin\\Quickicon\\Joomlaupdate\\' => [JPATH_PLUGINS . '/quickicon/joomlaupdate/src'], 'Joomla\\Plugin\\Quickicon\\OverrideCheck\\' => [JPATH_PLUGINS . '/quickicon/overridecheck/src'], 'Joomla\\Plugin\\Quickicon\\PhpVersionCheck\\' => [JPATH_PLUGINS . '/quickicon/phpversioncheck/src'], 'Joomla\\Plugin\\Quickicon\\PrivacyCheck\\' => [JPATH_PLUGINS . '/quickicon/privacycheck/src'], 'Joomla\\Plugin\\System\\Cache\\' => [JPATH_PLUGINS . '/system/cache/src'], 'Joomla\\Plugin\\System\\Debug\\' => [JPATH_PLUGINS . '/system/debug/src'], 'Joomla\\Plugin\\System\\GuidedTours\\' => [JPATH_PLUGINS . '/system/guidedtours/src'], 'Joomla\\Plugin\\System\\PrivacyConsent\\' => [JPATH_PLUGINS . '/system/privacyconsent/src'], 'Joomla\\Plugin\\System\\Shortcut\\' => [JPATH_PLUGINS . '/system/shortcut/src'], 'Joomla\\Plugin\\System\\Stats\\' => [JPATH_PLUGINS . '/system/stats/src'], 'Joomla\\Plugin\\System\\Webauthn\\' => [JPATH_PLUGINS . '/system/webauthn/src'], 'Joomla\\Plugin\\Task\\Checkfiles\\' => [JPATH_PLUGINS . '/task/checkfiles/src'], 'Joomla\\Plugin\\Task\\DemoTasks\\' => [JPATH_PLUGINS . '/task/demotasks/src'], 'Joomla\\Plugin\\Task\\Requests\\' => [JPATH_PLUGINS . '/task/requests/src'], 'Joomla\\Plugin\\Task\\SiteStatus\\' => [JPATH_PLUGINS . '/task/sitestatus/src'], 'Joomla\\Plugin\\User\\Profile\\' => [JPATH_PLUGINS . '/user/profile/src'], 'Joomla\\Plugin\\User\\Terms\\' => [JPATH_PLUGINS . '/user/terms/src'], 'Joomla\\Plugin\\User\\Token\\' => [JPATH_PLUGINS . '/user/token/src'], ];

ok - so that file is ok.

next question
do you have any authentication plugins on the site other than the three core plugins (cookie, joomla, jdap)

Nope.

ok - found the error I think

can you edit plugins\authentication\joomla\src\Extension\Joomla.php and add this line aywhere around line 15
use Joomla\CMS\Language\Text;

Error gone. It now says "Your access has been denied." And that is what should happen - forgot about that - because the site is offline. But you found the culprit anyway.

well actually the culprit is @laoneo and the change he made with #39624

the use statement was removed - I can easily create a PR to put it back but maybe there was a reason for removing it that I dont understand.

Thanks anyway for finding it :)

its part of the 12 steps

assuming that is was an error to remove that line I have created a PR to put it back #40518

Closing as having a pull request. Please test #40518 . Thanks in advance.