Now I checked it again, after some updates, and it works again. I'm back in accelerated graphics mode.
If you wondering, why I have problems at all: My laptop is a 1.6 GHZ dual core with VIA chipset and a nvidia 8400M graphic card. This turned to be a problematic combination several times with Ubuntu. Also the wireless lan adapter was unusable for a long time. Until an update at 9.10 made it work.
However, I'm glad now that I can benefit from the accelerated graphics for video and play some little OpenGL games again.