On the plus side, since I pinpointed it now to GTK3, I was able to install lightdm-kde-greeter to get non-GTK3 greeter for lightdm, and I can now log in to Unity 8 (which doesn't work with alternative display managers) which is the main test purpose of the machine currently.
Managed also to gdb into the python process before apport-collect crashed, looks similar: http:// pastebin. ubuntu. com/23268999/
On the plus side, since I pinpointed it now to GTK3, I was able to install lightdm-kde-greeter to get non-GTK3 greeter for lightdm, and I can now log in to Unity 8 (which doesn't work with alternative display managers) which is the main test purpose of the machine currently.