Ok if anybody experiences the same issue: I ordered both a 1080p (amazon basics) splitter and an AV converter - The AV converter was the problem.
Thanks for your answer, but unfortunately that didn't help. As soon as the HDMI2AV connects on output2 of the splitter, the res still drops.
I use a 4K x 2K UHD splitter - do you have recommendations on Amazon which one to use instead (also regarding HDMI2AV)?
I am experiencing a weird issue with my Ambilight (HDMI splitter -> HDMI2AV converter -> USB grabber -> RasPi3 -> -WS2801 LEDs).
If the HDMI2AV converter is connected to the HDMI splitter, the resolution of both outputs of the splitter is drastically reduced (but the Ambilight works - also able to set a constant color/effect via app, so grabber and SW should be OK).
If I connect 2 monitors to the splitter, both outputs are Full HD as expected. But as soon as I change the second output to the HDMI2AV converter, the resolution drops on both outputs.
So now my question is if anybody has had the same issue.
I strongly think this is a HW problem, I tend to think the HDMI2AV is the problem (although it shows a picture when connected directly to the AV input of the TV).
Or is the splitter the problem?
Do you have any ideas/hints?