I tested a bit more. It also works when I SSH into my Raspberry, and reboot it when the AV receiver is already on. Then I have to open an SSH session again, so the script actually runs. What confuses me, is that I had it working before and I don't feel like I changed anything.
It comes down to these two problems:
- I have to reboot the Pi after turning on the AV receiver so it actually loads the /dev/video0 driver(?)
- I have to SSH into the Pi after the reboot to make sure the bash-script that should run automatically on startup actually starts. When I connect via SSH I see hyperion is running and I get the output there. So I can't input any commands either.