WiiM Sound as default as WiiM Ultra default audio output

[Burnside]

I used fixed volume output. Maybe something is wrong with my optical output resolution settings, because i don’t have a clue how it should be set up. Even the test tone didn’t work

Those settings don't look unreasonable for input to an Ultra. I'd maybe try 16/44.1 as that's still fine for AirPlay. I'd also try another optical lead before raising a ticket to WiiM via the more/feedback section in the app to see if they can detect any issue from your device logs.