Obviously openSUSE-SU-2017:0290-1 is the reason: I was able to reproduce the bug on a second system, but only after updating tigervnc from 1.6.0-3.1 to 1.6.0-6.1. With the old version everything works fine. By the way, in my last test, vnc allowed connecting twice and rejected from the third connection on. So if you are trying to reproduce the bug, please maybe try more often.