I think the problem is my center speaker, B&W CC6.
When I turn down volume on this speaker, then I have no problem.
I have changed the reciever setting from 8 to 6 ohm or higher. (I have 8 Ohm speakers, according to their specs.)
This has eliminated the problem so far.
My reciever has the YPAO function, and apparently it cannot measure a defect on the speaker, and the frequency curve of the center speaker also seems to be ok.
So, I think the problem is, that the center speaker is not 8 Ohm, but more likely 6 or 7 ohm, which could give the reciever problems, if it is set up for 8 ohm or more.