It turned out that deleting ~/.config/octopi resolved the issue. Sorry for the false alarm.
To proof that I am not crazy here is the output of octopi before that deletion:
$ /usr/bin/octopi
Using a variable-width font in the terminal. This may cause performance degradation and display/alignment errors.
Using a variable-width font in the terminal. This may cause performance degradation and display/alignment errors.
Speicherzugriffsfehler (Speicherabzug geschrieben)
The last line means Segmentation fault (core dumped). So something in ~/.config/octopi made octopi crash (no further inverstigation 
).
Thanks for the help!