Since installing the new app I lost my sub, it's just not possible to add to groups due to "connectivity issues". The system still sees the sub as present, so no idea what the problem is.
After about an hour of tinkering or so I gave up, deleted the new app and sideloaded the old app (16.1). And guess what, I added the sub with no problems…
Unbelievable how a broken product like the new app can get released in the current state. Even if this gets fixed, the reputational damage to Sonos will definitely have impact on sales.
So anyways, I can highly recommend sideloading the old app and turning off auto updates.