Issue Plesk admin panel not accessible

[solutionsnewtown]

Server operating system version

Debian 10.13

Plesk version and microupdate number

Plesk Obsidian 18.0.60.0

Hi everyone,
It seems that our Plesk server (hosted with 1AND1/IONOS) is having an issue out of nowehere. We we're able to access the plesk admin portal, couple days earlier and now we got an ERROR 500 when we try to access it.

1. - We've try a plesk repair
   1. Unfortunately, exit 255. we've followed the KB online but still same issue.
2. We've try a plesk installer, full update
   1. We were able to update but at end we goth a everytime we got that (same error as a stack trace get-login-link):

lstat("/opt/psa/admin/plib/vendor/plesk/zf1/library/constants.php", 0x7fff3eef1400) = -1 ENOENT (No such file or directory)
access("/var/parallels/feedback-disabled", F_OK) = -1 ENOENT (No such file or directory)

We're out of ideas to fix this situation.
The service itself for the client is working correctly but unable to manage it without the panel.