When CUT2's mutator and UT2004RPG are loaded and running together on the same server certain weapons do not function. They can be picked up and readied, but they show an ammo of 1 and will not shoot. This is only with specific weapons -- from what I can tell, the grenade launcher and the Claw 2 are affected by this. Additionally, guns that switch ammo types can't switch on the fly. The MUG will shoot but only switches ammo if you run out of one.
UT2004RPG works just fine with most other weapon mutators (like WeaponLord) but seems to have trouble with this one.
Because I don't know where the problem lies, I've posted something similar over at the UT2004RPG forums as well.