UpdateGamma crash at launch

On my new iMac with a GeForce GTX 780M running Mavericks, Killing Floor crashes on launch more often than not. Every time, the crash log ends with:

Quote:

UOpenGLRenderDevice::UpdateGamma <- UOpenGLRenderDevice::SetRes <- USDLViewport::TryRenderDevice <- USDLViewport::OpenWindow <- UGameEngine::Init <- InitEngine

What confuses me especially is that it doesn’t always crash, even when nothing changes between attempts. Please help.