Now the big question, is it a general php issue, is it an issue how plesk compiles php or is it something related to the virtualization platform.
Ans suggestions for further test are welcome.
In that case, if the problem is caused by PHP version from Plesk, you can draw your attention to this KB article, which describes the causes and possible solutions.
I had the same issue after the NC update on PHP 7.3. Could not login.
The solution was to change the Redis / Memcache settings. I changed for Redis to APCu this line:
‘memcache.local’ => ‘\OC\Memcache\APCu’
Then cleared the Redis cache. You have to wait to have this change effective. But it worked som time later.
PHP 7.4 gave also trouble. Not tried this again yet. It could be that these problems are related to IPv6. There is other code generated then with IPv4 connection.