Another day went by and one step forward was made!
I removed the following entries from config.php and restarted apache.
âmemcache.localâ => â\OC\Memcache\APCuâ,
âmemcache.distributedâ => â\OC\Memcache\Redisâ,
âmemcache.lockingâ => â\OC\Memcache\Redisâ,
I am now able to logon and the system page shows me PHP 7.4.8!
Ofcourse the overview page is now complaining now caching is activated, which I will not have to look at. But at least I was able to narrow down, where the issue is.