Thanks Digi who found this in the official forum
Mark Rein posted the following:
Folks,
Daniel Vogel informed me that he unknowingly accidentally broke Voodoo3/5 compatibility in the demo. We"re fixing it and there will be a patch that addresses that plus a few other major bugs we discovered. Keep watching the Official Unreal Tournament 2003 Site for news of its release.
We are sincerely sorry for this. Daniel claims it was just one, itty-bitty, single little line of code that caused it to break. I"ll make sure that I personally play the gold master for the full game (when it"s done) on Voodoo3 and Voodoo5 to make sure it doesn"t happen for the retail version.
Thanks for your patience and again, our apologies.