Thanks, RoryOF! That did the trick. Overinstallation did not work. The US-version was not recognized as different from the .de version. So I installed Revo, deleted User, and reinstalled the US-version. And voila: Calc works like new

I originally thought the different versions were about user language of the program only.
By the way: Anyway to change the user language of the US version into German? I didn't find any option.
Better to say, I find the option (user interface) but only Englisch (US) is available there. Are there language packages for user languages, too?