I'm assuming your using the original retail build. This error has been corrected with the new patch. You can go to www.unrealtournament.com
download it from there (sorry I can't give a link to direct download).
Anyway, this error has been documented several times at bugzilla.icculus.org.
Also, when applying the patch make sure the ucc-bin and ut2004-bin files get copied (i had to do them indiviually). I had an assertation error because when i copied they didn't get copied when the rest of the files did.