@joomdonation
Confirm Actual Result. Updated from 4.1.4 to Nightly Build 4.2.0-beta1-dev.

An error has occurred. 0 Class 'Joomla\Plugin\Actionlog\Joomla\Extension\Joomla' not found

But now, Frontend and Backend are corrupted.
Can I solve it via phpMyAdmin somewhere (special Plugin Table)?

@ChristineWk You can delete the file administrator/cache/autoload_psr4.php manually to get the site running again. However, to test the PR, you will need to setup another Joomla 4.1 website. The purpose is confirming upgrading to new package won't get fatal error anymore

@ChristineWk You can delete the file administrator/cache/autoload_psr4.php manually to get the site running again. However, to test the PR, you will need to setup another Joomla 4.1 website. The purpose is confirming upgrading to new package won't get fatal error anymore

OK, Done. Thanks for the hint :-)
It shows now: 4.2.0-beta1-dev and: Database: 4.1.3-2022-04-08: After this: DB updated.
Another 4.1 I don't know yet.

From what I remember that cache file is supposed to be deleted and rebuiult during the update process

found it #36094

@brianteeman That PR only solved the issue when new extensions are added during update process (like Schedule extension which we added in 4.1). For 4.2, we converted some plugins to new structure, so the namespace map must be re-generated earlier than that, otherwise, after the extraction process, we will get fatal error immediately and the upgrade could not be completed.

I have tested this item ✅ successfully on 7aaebc4

Hint for other testers: For updating to the 4.2-dev nightly build without the patch (step 2 in the instructions), I had to download the update package from here https://developer.joomla.org/nightly-builds.html and use "Upload & Update" because the update site XML for the 4.2-dev nightlies contains a wrong download URL. See joomla/update.joomla.org#250 .

I have tested this item ✅ successfully on 7aaebc4

Confirmed the error message and tested successfully in Wampserver 3.2.9 and PHP 8.0.15 using the update package created for this PR.

#
#<?php die('Forbidden.'); ?>
#Date: 2022-06-06 03:43:55 UTC
#Software: Joomla! 4.1.4 Stable [ Kuamini ] 24-May-2022 16:00 GMT

#Fields: datetime	priority clientip	category	message
2022-06-06T03:43:55+00:00	INFO 127.0.0.1	update	Starting installation of new version.
2022-06-06T03:44:15+00:00	INFO 127.0.0.1	update	Finalising installation.
2022-06-06T03:44:15+00:00	INFO 127.0.0.1	update	Start of SQL updates.
2022-06-06T03:44:15+00:00	INFO 127.0.0.1	update	The current database version (schema) is 4.1.3-2022-04-08.
2022-06-06T03:44:15+00:00	INFO 127.0.0.1	update	Ran query from file 4.2.0-2022-05-15. Query text: CREATE TABLE IF NOT EXISTS `#__user_mfa` (   `id` int NOT NULL AUTO_INCREMENT,  .
2022-06-06T03:44:15+00:00	INFO 127.0.0.1	update	Ran query from file 4.2.0-2022-05-15. Query text: DELETE FROM `#__postinstall_messages` WHERE `condition_file` = 'site://plugins/t.
2022-06-06T03:44:15+00:00	INFO 127.0.0.1	update	Ran query from file 4.2.0-2022-05-15. Query text: INSERT INTO `#__extensions` (`package_id`, `name`, `type`, `element`, `folder`, .
2022-06-06T03:44:15+00:00	INFO 127.0.0.1	update	Ran query from file 4.2.0-2022-05-15. Query text: UPDATE `#__extensions` AS `a` 	INNER JOIN `#__extensions` AS `b` on `a`.`element.
2022-06-06T03:44:15+00:00	INFO 127.0.0.1	update	Ran query from file 4.2.0-2022-05-15. Query text: DELETE FROM `#__extensions` WHERE `type` = 'plugin' AND `folder` = 'twofactoraut.
2022-06-06T03:44:15+00:00	INFO 127.0.0.1	update	Ran query from file 4.2.0-2022-05-15. Query text: INSERT IGNORE INTO `#__postinstall_messages` (`extension_id`, `title_key`, `desc.
2022-06-06T03:44:15+00:00	INFO 127.0.0.1	update	Ran query from file 4.2.0-2022-05-15. Query text: INSERT IGNORE INTO `#__mail_templates` (`template_id`, `extension`, `language`, .
2022-06-06T03:44:15+00:00	INFO 127.0.0.1	update	End of SQL updates.
2022-06-06T03:44:15+00:00	INFO 127.0.0.1	update	Deleting removed files and folders.
2022-06-06T03:44:19+00:00	INFO 127.0.0.1	update	Cleaning up after installation.
2022-06-06T03:44:19+00:00	INFO 127.0.0.1	update	Update to version 4.2.0-beta1-dev+pr.37990 is complete.

RTC

Thanks everybody

Cross posting #38474 for visibility - Joomal 4.2.0 Upgrade bricks sites...

I have a related issue maybe?

I have removed the file suggested above "@ChristineWk You can delete the file administrator/cache/autoload_psr4.php manually to get the site running again. However, to test the PR, you will need to setup another Joomla 4.1 website. The purpose is confirming upgrading to new package won't get fatal error anymore" But that has not solved my issue.

Any suggestions : Live site is at this URL (I also stupidly did not do a backup!)
https://www.cheshuntrpc.co.uk/

Thanks
Numpty Mark from UK

set debug mode on and error reporting to development in configuration.php to get a stack trace - this is a PHP 8 issue and unrelated to autoload_psr4

Thanks PhilETaylor. Where do I do that please?

Community Support is best obtained through the https://forum.joomla.org and not here.

You can edit /configuration.php with a editor and change $debug = false to $debug = true and $error_reporting = 'none' to $error_reporting = 'development' and then save the file back to your site.

Sorry, I didn't know I was in the wrong place. I will do that as suggested and also look there. :-)

Dears, I have new problem, please help
Class "Joomla\Plugin\Authentication\Joomla\Extension\Joomla" not found

@rkostanyan Try to delete the file administrator/cache/autoload_psr4.php from your site. That should address the issue.

Problem is not resolved:(

@rkostanyan If so, I don't have any other idea. You might need to have a developer to look at your site to see what's actual reason and get it sorted. Without accessing to your site, it's hard to know the root reason of the error