There is no debug log but if you launch Knossos in debug mode ("KN_DEBUG=1 knossos"), the debug messages are printed on the console.
The dbus error happens on my machine, too, but it doesn't cause a crash. I haven't figured out what causes it since I'm not using it directly.
The second message is most likely the cause for the crash. In case it helps, here's the code that creates the settings window
EDIT: Managed to reproduce the bug and track it down. If I call SDL_InitSubSystem(SDL_INIT_VIDEO), a segmentation fault occurrs in QPen::QPen() but the stacktrace leading to the crash contains no calls from my code nor SDL. I guess something weird happens in Xlib or XCB because both Qt and SDL2 are trying to use it.
This isn't the first time that this code leads to crashes. Since I only need SDL2 once to get a list of supported resolutions and joysticks, I'll launch seperate process for that which doesn't use Qt.