Overnight Internal Server Error - bound to php7.4

Nextcloud version (eg, 20.0.5): 20.0.8
Operating system and version (eg, Ubuntu 20.04): Debian 9.13
Apache or nginx version (eg, Apache 2.4.25): Apache/2.4.25
PHP version (eg, 7.4): 7.4 but stopped working overnight, rolling back to 7.2 works, but cant login to users anymore then (different hashing?)

The issue you are facing: Overnight our nextcloud stopped working because of an “Internal Server Error”. The apache2 error.log is not showing something strange, but I cant interprete it and everything I found on google didn’t seem to work.
I had php7.4 installed and it was running flawlessly for a long time. I experimented a lot the past hours. Rolling back to php7.2 seems to get the server back up, but no user is able to log back in so this isnt a solution as everyone will loose his file due to encryption.
If I forgot to provide something relevant, please tell me.

Is this the first time you’ve seen this error? (Y/N): Yes

Steps to replicate it:

  1. apt install php7.4 && a2dismod php7.2 && a2enmod php7.4
  2. systemctl restart apache2
  3. open nextcloud and experience Internal Server Error

The output of your Nextcloud log in Admin > Logging:

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

The output of your Apache/nginx/system log in /var/log/____:

In the process of writing this I had a few more ideas to try and actually solvedit.
installig php7.4-sqlite and php7.4-mysql got me a little further to getting occ running.
Afterwards I went through the nextcloud php dependencies list and installed them all (most of them alread were but just to be sure).

Then I looked at php -m and noticed that php7.2 got memcacheD installed and php7.4 memcache, so I did apt install php7.4-memcached && apachectl restart

Not its back up and running