Out of interest, do you run Roundcube?
If you look at this thread: Question - Plesk Obsidian and PHP 8 compatibility
You'll see, that with effect from Plesk Obsidian 18.0.49, Roundcube runs on PHP 8.0. So ...IF... you fall into the criterea that's covered in that thread, then that might be a cause of the issue that you've experienced. From our own past experience, we had a similar error (but on earlier PHP and Roundcube releases).