### Steps to reproduce the issue

1. Both Languages installed and published. Both Content languages created and published (German is default language)
2. Main menus for ALL, en & de have their own default homepages. Menu module for ALL is unpublished. Both language menus/modules are published
3. Multilingual Status is published, shows no issues.
4. Language filter is active - "remove url language code' is set to 'yes'
5. Language switcher is active and published
6. Template YP - after switching to Cassiopeia the problem remains

Expected result

Site switches to English with ..../en and to german without de

Actual result

Switching from German to English works
Switching back to German gets an 404 error page not found

System information (as much as possible)

PHP 7.4.23, Joomla 4.0.3
The problem appears on a site hosted by IONOS. The same problem is with a site hosted by Rochen
A copy of this page transferred to Host Europe works.

Additional comments