What are the causes of lockups with Debug on

Blitz always runs apps in a window when debug is turned on. Maybe something to do with that. Perhaps your driver/Graphics card doesn't support 3D in windowed mode, or something. Check out these various commands:
CountGfxModes3D

GfxMode3D

GfxMode3DExists

GfxDriver3D

GfxDriverCaps3D

Windowed3D