Thank you Peter, I tried the official support, but he linked me to some posts which I already knew an can be found in google. So I was forced to make a new installation of linux and plesk on my webserver and backup the domains. Anyway, maybe this was the better solution at least, if one issue comes with an update, it can turn out to be a never ending story of issues, so I am ok with the new installation.
If somebody is reading this, I repeat the mantra here: If you make an mayor update of plesk, always backup and be prepared and able to make a new installation from scratch. Otherwise you are f*cked up.