Just let me clear a few things up:
1. I am a consumer, not a developer nor a beta tester.
2. I bought a board with an internal graphics accelerator which was advertised as "accelerating HD decoding", though not explicit for Linux, fore a HTPC.
3. A few month later AMD/ATI discovered they cannot enable HD acceleration in Linux because of their insufficient design.
4. Don't get me wrong, Nvidia is not better. 5. Now my Media server is working.
6. The trouble with my girlfriend is less, because Greys Anatomy doesn't stutter or flicker.
My offer is still valid to look for clues why this driver failed to work. If someone can tell me what to try or where to look.
It is a bit unclear what the exact reason is... might have something to do with dual-head. It seemed X was not able to start sooner when I changed some overscan options of my connected TV. But with more testing it appeared that X didn't start either when I didn't change any options at all... So I went back to 8.9 and everything works again without problems.