Hey @juist,
Welcome to the Sonos Community and apologies for the late response.
Sorry to hear you are unable to group your speakers as desired. The only time I have come across this particular error with grouping, was when I had a mismatch in firmware of the different devices - although the Sonos App should prompt for an update if any Sonos devices are not on the latest firmware. Could you check for updates in your Sonos App please → Update your Sonos speakers
If this is only an intermittent error, it might be worth doing a reboot of your network and Sonos devices.
Should all fail, I suggest you reach out to our technical support.
Hope that helps!