Video problem in Nexuiz

This happens when running the open source Nexuiz game. The screen is black but it flickers (every second or so it shows 1 frame and turns black again).

Judging from what I can see, there is no garbage on the screen. Also, when I load a level through the console, the loading screen appears, where the problem doesn't appear. As soon as the level loads the problem is back.

I tried doing the same in a Windows XP virtual machine and there is no problem there, though the video driver in ReactOS is the default VGA. Changing resolution to the minimum has no effect.

Attached a debug log.