There is a brush with too many sides. If you have not created such a brush, it could be from a custom pk3 in your MOHAA\main folder. I recomend that you remove all custom pk3s while you are developing a map.
I agree. A few steps you need to take before going into the wild blue yonder of mapping in mohradiant is to remove custom .pk3 files if you have them in the main folder. Radiant tries to load them up when you start it up. Sometimes this causes a crash.
One other thing. If you have Spearhead on your computer, and do not have the plug in for it in radiant then you will encounter problems as well.
Instead of removing spearhead if this is the case just go into MOHAA folder under program files and find the folder that says "mainta" rename it to say... "painta". BUT WARNING... (didn't mean to freak you out)... you MUST rename the folder back to "mainta" in order to play Spearhead again.
Hope that helps.