I also tweeted to Plesk support about this.
Here's the issue. There's some kind of discrepancy between passwords for accessing the mariadb and the Plesk installation. I don't know what I did wrong, but the only way I can solve the issue is by constantly manually updating the admin password at command line after every Plesk update. I can also "break" Plesk by trying to go to phpMyAdmin from within the Plesk panel. If I do that, then I get the admin@localhost denied error even on the panel...
Passwords problem
Here's the issue. There's some kind of discrepancy between passwords for accessing the mariadb and the Plesk installation. I don't know what I did wrong, but the only way I can solve the issue is by constantly manually updating the admin password at command line after every Plesk update. I can also "break" Plesk by trying to go to phpMyAdmin from within the Plesk panel. If I do that, then I get the admin@localhost denied error even on the panel...
Passwords problem