Hi,
that is what I done.

I have flashed the same clean image to the two boxes. The one that has the hdmi ok showed the bootlogo immidiately after the switch one and the video configuration settings at the end of the booting process on the tv screen.
The one that has the problem also shoed the bootlogo on the tv screen immideately after the switch on, but then "no HDMI signal" on the tv screem during the booting process- At the end of the booting process I can see the video configuration setting on the dreambox display but nothing on the tv screen.

so I'm 100% sure of a software/image that is exactly the same and has exactly the same configuration.