Spring RTS Engine

I used to run the propreitary drivers, but had all kinds of problems when trying to play another game called Tremulous. It is a quake3 engine based game, and the lightmaps would lag the heck out of my video for some reason, but switching to opensource radeon drivers fixed the problem. Well, at least...

Just for reference, I have an ATI Radeon 9600 PRO card and I'm using the latest opensource drivers.

I have this exact same problem/error, but only if I try to run a mod besides XTA. Any ideas ? It must be game binary or data related, not graphics driver, since I can play XTA mod just fine without any problems using AAI ai.