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 am experiencing a weird issue with my Ambilight (HDMI splitter -> HDMI2AV converter -> USB grabber -> RasPi3 -> -WS2801 LEDs).
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