The HDMI1 is the output that goes through the Marvel VSP, while HDMI2 is from the MTK processor (i.e. the original output). Which TV are you connecting to? I believe the HDMI1 tends to have slightly more compatibility issues and has been refined over various firmware releases (but you had upgraded to the latest FW).
First of all, thank you very much for your readiness to discuss my problem with me.
Meanwhile I've got some new facts and possible traces.
When I separated disks on which the malfunction tended to occur, I realised they all
were 1080i(!). Since my player resolution setting was 1080p, I first concluded that the conversion from 1080i to 1080p was the problem. However, further tests showed that
even player settings 1080i or 720p didn't remove the problem. Although. the problem occured less often (approx. once in a concert).
My TV is the Panasonic 37'' plasma. It can take and handle 1080p input, but the output in pixels is less than full HD (approx. HD Ready).
Yes, I upgraded to the latest FW.