Hello Wolfgang,
Unfortunately AD9880 support is limited to the docs already posted. It's an NRND part that will probably go obsolete sooner rather than later.
The only issues we are aware of center around 1080p rather than any HDMI version issue as this was a pretty uncommon format when that chip was designed. You'll find several threads with suggestions for that if you search. Notably increasing the core voltage from 1.8V to 1.9V can solve some problems. That's really all we can offer.
The current replacement for AD9880 is ADV7842.
Dave