Thanks for the tip but unforunately it didn't solve my problem.
Restarting was one of the first things I did when I discovered the problem,
didn't help thou. The keys are all working properly, none is stuck, sticky or
anything like that.
I've found something new thou: If I push and hold a non-working combination,
for example CTRL+F, I get after awhile a beep every second that doesn't occur
if I push and hold a working combination, like CTRL+X or CTRL+A