Php8 made nc20 disfunctional, revert to php7.4 impossible

Problem:
nc20 worked fine on my Ubuntu 18.04
I installed all updates with apt-get update && apt-get upgrade
This also upgraded PHP 7.4 to PHP 8.0
Now my nc20 doesn’t work anymore.

Symptoms and history:
Now my nc20 doesn’t work anymore. As I read, nc20 does not support php8: It just shows an incompatibilty message from versionchech.php If I modify that file to bypass the version check, just a plain white page is shown and loads of errors concerning database accesses are logged.
Calling a simple phpinfo.php website confirms my Apache and my PHP8 would run fine, nc20 just can’t handle it. So back to PHP 7.4!
If I undo the modification in versioncheck.php and then
a2dismod php8
a2enmod php7.4
service apache2 reload
service apache2 restart
my nc20 still won’t run! It shows a error 500 internal server error message and I can’t find something inside the error logs.
Another call to the simpel phpinfo.php page confirms my Apache and my PHP 7.4 would run fine, just my nc20 can’t handle it anymore (it did so before!).

My wish:
Either make my installation of nc20 able to run again with PHP7.4, how?
Or (even better): I have read that nc21 will support PHP 8 somewhere here in the forums. Is this confirmed? When will nc21 to be expected? And how can I upgrade my nc20 to (a php8-supporting) nc21 when the instance doesn’t run and therefor I cannot start the updater???

Try to remove php and reinstall version 7.4. Apt should be able to do that.
Maybe this helps: How to Install Specific Version of Package using apt-get

This worked. But now in “Apps” I only see “my apps”, “activited apps” and “inactive apps” (and the app packets). But I don’t see the App Store / App Market, and I don’t see an option to update the apps! How do I get back the function to update and install the apps within nextcloud?