Thanks a lot for your help!
I deactivated the squeezebox server on one device but the issue persists.
The issues evolves as follows:
I select the two synced devices in spotify.
For 4-5 seconds both devices play synced music.
After that the device that is not running the squeezebox server stops playback.
Shorlty after that the device running the squeezebox server stops, too.
With the next song the whole process repeats itself.
Is this a known issue?
Many thanks in advance and best wishes