Thanks for your quick response Hias.
Ok, the HDMI audio device is blocked. That is also what the error says. But I still have these 2 questions:
Why does squeezelite gets acces to HDMI output and works fine when I start it manually without (...)&. Is it just because of the reason that is not a background process? If yes, why did squeezelite worked as a background process with libreelec 9.2. There I had squeezelite running and also Kodi audio at the same time over HDMI. Worked perfectly without configuring pulse audio. Just the normal HDMI output was used.