Thank you for your quick reply.
I have updated my version to 3.1.2 which didn't help unfortunately. The problem occurs on my MBP, where I am using Aperture for years now. I am using my same laptop screen. I try to describe the problem a little further...
I only have RAW (*.NEF) files in my library. Most of them are with adjustments. Now here is the problem: When I open a picture with a adjustment, it show as usual in the viewer or fullscreen. But JUST for one second. Then Aperture automatically changes the adjustment. Very weird and never happened before. This happens to all the adjusted pictures. After this one second the picture changes (not to the original), though the adjustment doesn't change.
Could it be a RAW update?