Okay, after some hours of searching I finally found the problem!

I had to go into the Xorg config file, change the color depth from 24 to 16, reboot, and now it's okay - I'm still gonna try if 16 is better or 32.

I think that's more likely a problem of PlaneShift than my Xserver, is it?

I have the same problem - flickering, and the window is split into to triangles flickering differently.

The problem:
can't have anything to do with the Ubuntu, since I have the same problem, with Debian Unstable.
can't have anything to do with graphics drivers for something special (because I use onboard graphics and I never had anything special to do) (but still, I'd like to know what installation program you're talking about?)
no problem of speed, because slow speed still doesn't flicker.

I continue to have this problem ingame as well, appartently always when the screen changes (when the screen loads and nothing moves, it doesn't flicker) - some problem with refresh rates??
I'd be happy if someone could help, since PlaneShift also doesn't work on my Mac (but with a crash a little bit harder than this here...)