Try unplugging your speakers from power for a couple of minutes and reboot your router and phone. Then try grouping again.

This is the “shutdown everything and reboot” solution.

I very much prefer to understand why it happened, what caused it to happen, and how to improve the system / setup.

It most likely happened due to your router losing track IP address allocation.  This can happen for numerous reasons and Sonos is particularly sensitive to this.  A reboot of the router and all products it addresses will rectify.  If your router allows you can select a range exclusively for Sonos, that will help.

Also if this happens frequently you can raise a diagnostic on the app and post it here.  Sonos support than then see what issues you may be experiencing a suggest remedies.

If the problem is IP address allocation then the solution is to get your router to refresh the addresses.  This is achieved by turning everything off and on again.

If it is an address issue you can almost always solve it by assigning static/reserved IP addresses for your Sonos followed by powering all down, rebooting the router to flush the old addresses and then powering back up one at a time, Boost/Bridge/wired first.

Your router’s DHCP page should make this fairly simple.

I have a reserved ip assigned to each sonos device. The reserved ip address for each device has not been changed for a long time. Also, if it is an ip address issue on the Play:1, it should also affect grouping from another sonos device (Test Case 2).

The key difference I see is in the topology where the Symfonisk are connected to each other (both as a stereo pair and via sonosnet), while the other 2 Play:1s are connected via the Beam. Both wired Symfonisk and wired Beam are on the same switch. I suspect this could be related to the network, but can’t figure out what.

I haven’t had the time to test it further by disabling one of the wired devices so that SonosNet is not split. In the end, I’ve decided to just wire the affected Play:1 (wiring the other Symfonisk is not an option). I’ve also disabled SonosNet and put the other devices on wifi.