We had same problem during update from Ubuntu 16 to Ubuntu 18. For some reason Plesk was no more part of our APT sources anymore and older plesk version was deleted during migration, because of mysql dependency.
(Also, for some reason, including "psa" database, which you must restore from a...
Same here, but we was able to fix.
We found related files with this command:
find /opt/psa/admin -name "CustomButtons.php" -ls
This give us the extension, which was updated recently. In our case "google-authenticator".
I uninstall from CLI and was able to access Plesk again.
A question would...