Okay I solved the problem by doing a backup with the backup addon (after the problem occured), a fresh Libreelec install and restoring everything except the config folder and guisettings....so I "only" had to do all the settings again.
Anyways today the issue came up again and after some troubleshooting I could definitely identify the snapclient add-on to be the source of the problem and not the audio profiles add-on.
I was setting up a snapserver/client when it happened and I think I installed these add-ons as well, when it first happened. So the problem must have only been solved because some add-ons don't work properly after restoring a backup and have to be reinstalled.
Uninstalling the snapclient and rebooting brings back all the lost audio output devices.
After doing some research about how to get the snapserver/client running properly I found this thread
Snapserver/Snapclient does not work on RPi4 with LE 10.0.1 via HDMI
maybe a problem someone else had already solved: Today I've been putting an RPi4 with LE 10.0.1 into action on a new Marantz NR1711 connected via HDMI0. With Snapclient deactivated on this machine, I can see via Settings -> System -> Audio -> Device the following entry which I picked: "ALSA-vc4-hdmi0 (vc4hdmi0), MJI marantz-AVR on HDMI". So far anything works like charm.
Since this device is in the living room and I do have an additional RPi3 in the kitchen/dining room running a…
which describes the same Problem
So the problem seems to already be there for a long time.
Isn't there anyone with a running snapserver/client setup, who can give instructions how to set it up correctly and how to solve the audio issue? ...Please?