Thx a lot for the quick reply and warning recomendation!
I reset the database with a backup. The very strange thing, then was:
In my (empty) “staging” phpMyAdmin account, I could upload the SQL dump without any issues. After having uploaded the – very same – dump into the (empty) productive account, rows with option_id = 0 in the _options table appeared. Guess what – now, the staging is the productive account.
Who knows. This thread’s issue is solved.