It seems, that I just have to restart the X server, to get things working again.
Although this seems to be another problem. In some circumstances restarting X will result in a black screen, while the computer tries to restart X with the following messages added to the kern.log file
[13589.026019] NVRM: RmInitAdapter failed! 0x12:0x2b:1671)
[13589.026029] NVRM: rm_init_adapter(0) failed
The funny thing is, that everything works quite nice, as long as I don't start anything else. For example I can play a game right after the X login. But if I start another program, like Firefox and/or Thunderbird, things will go haywire.