Issue can be closed.
because I'm usually lazy ;-) I have used the simplest solution.
Instead of thinking much about possible problem, I just copied sudoers file from other openSUSE installation and overwritten the wrong one.
Afterwards I executed this:
chmod 0440 /etc/sudoers
...and now it works fine again. I have to add some entries there, but thats fine for me.
I'm still thinking what could get wrong, because I never edit sudoers file directly (via visodo). I only use the option to update the file via YaST2 (openSUSE manager).
I assume it was YaST, who destroyed the file.