Ganni1987: I'll download Xubuntu 15.04 and test this out myself, will tell you step by step what I did.
august: fwiw it may come down to driver and kernel driver changes the GalliumOS team has made to Xubuntu.
Placing a temporary solution in a new post:
Someone on a forum posted a suggestion to switch the game to software rendering mode. Looks kinda crap basically but the game runs and I've also managed to run it on Xubuntu 15.04.
Here's what you have to do:
1) Go in your "home folder/.loki/ut/System/"
2) Open up 'UnrealTournament.ini'
3) In 'Engine.Engine' section (first 3 lines), edited these lines to look like this:
GameRenderDevice=SDLSoftDrv.SDLSoftwareRenderDevice
WindowedRenderDevice=SDLSoftDrv.SDLSoftwareRenderDevice
RenderDevice=SDLSoftDrv.SDLSoftwareRenderDevice
The game will now run in software mode. The issue seems to be related to the graphics renderer. Unfortunately for now this is all the help I can give you.
Update: I've installed Xubuntu 15.04 on another system with an Nvidia GPU + Proprietary drivers and the game runs without issue straight out of the box. So far I can only conclude that this game doesn't run on Mesa drivers.
At this point the only advice I can give you is to run it in Software Mode as I explained at the beginning of this post, or run it in Wine using an OpenGL renderer.