(partially) solved the problem.
Updated our test system to checkmk 2.0 with same behaviour.
Found out, that one of our maps seems to be the problem.
I removed all .cfg files under etc/nagvis/maps, including Automap.cfg. After that I could start nagvis and create a new Automap.
Moving cfg files back in place until nagvis stopped working again. One map was causing that no other map or nagvis itself could be opened. I will try to recreate the map in the next week(s).