Hello Everyone 
So, up until a while ago everything worked great. My setup is the following:
Raspberry Pi 4b (4GB RAM) outputting to HDMI0 (the port next to the USB-C) into a HDMI 2.1 Switch that Feeds a HDMI 2.1 Splitter from which one output goes to the TV (Hisense 55E7NQ, 2024) and the other one into an HDMI grabber that goes into HyperHDR (also running on the Pi). I remember when I first set it up I had some problems to get a picture out of the Pi but then it worked and I had no issues over the past months until about a week ago when I reinstalled LibreElec due to some plugin problems.
I can't get any image out of the HDMI0 port. The TV recognizes that Kodi is indeed connected there (in the HDMI menu) but tells me "weak or no HDMI Signal". I tried restarting everything multiple times, changed the startup order but to no avail. Switching to HDMI1 on the Pi and rebooting immediately fixes the issue and I had picture again with the setup (but no HDMI CEC and from what I read it's very tricky to get that working). If I remove the HDMI Switch and plug the Pi straight into the splitter, the first HDMI port works as expected. Weird thing is, my PC feeding the same HDMI switch before the splitter has no issues at all and I even bought a different HDMI 2.1 switch yesterday (one that has its own power supply, just to be sure) but the exact same behavior occurred. No output from the first port, second one and PC work flawlessly.
While trying to get this working, I had image for a short while but when I restarted everything the problem reappeared and now I can't reproduce it.
Any Idea what could be the problem? I don't think the port can be physically defective since there's no problems when it's connected to the splitter directly, even when I move the cable around.. Thanks for your help!