I think I fixed it.
my mistake: I tried to workaround this bug:
https://bugs.x2go.org/cgi-bin/bugreport.cgi?bug=1551
...by editing /etc/cups/cups-x2go.conf
that's why I ran into this bug (I guess):
https://bugs.x2go.org/cgi-bin/bugreport.cgi?bug=890
so I now leave /etc/cups/cups-x2go.conf unchanged and instead edit /usr/lib/cups/backend/cups-x2go (remove -c setpdfwrite)
... and remote printing finally works for me now.