It's the 2nd cd's textures. If the server doesn't have them installed, the other player's movements are not animated. Also, dead bodies are not animated, they appear to be standing there. It could also screw up your voice binds by switching them to another character.
I bet it's fine single player.
I personally have a 2nd version of UT installed. I use a cherry version for online play, and for practising/single player I use the compressed textures on a 2nd install because of the problem you've listed.