thanks for your reply.
According to it i tried to create a screenshot. But for whatever reason, opera started up just fine.
I restarted opera in the last couple of days, but it never changed (and therefor i think was not the solution of the problem).
The only thing i can remember i did was to press F11 to switch to full screen mode and pressed F11 again to switch back to normal mode. At this time the screen looked like befor. Than i closed opera. But i did not restart it again. Probably pressing F11 and closing opera browser already solved the problem.
If the problem reoccurs i will post a screenshot here, so if somebody else is running into the same problem, he might already have a solution for it.
Thanks a lot for your help.
Best regards, aurikus