Actually it was (kinda) fun to fix the board. I haven't worked with mySQL databases in a while and it was good to dig deep and write cryptic commands in the shell, hoping at the same time not to grill the board (I've made a backup of the database before I came along with the terminal commands... just too be safe).
Somebody asked: How can this happen? Actually, it should not happen. But it did. Obviously the vast amount of users on the board right after the SuperMeet was just too much to handle for the server. Luckily it could be fixed... and now I the tools installed to get much faster access to the database in case something like this happens again.