I have exactly the same issue, didn't notice until a few days after the app update. I have a second set up of soundbar, mini sub and 2 Play 3’s no issues.

I have rebooted router, factory reset eras, deleted and reinstalled the app - no result. First line help suggested the same and then to reset the app - didn't work - told me I needed second level support but have given up after waiting 45 minutes. Sounds like they cannot help either.

Keep in touch if we get a solution…...

I have also just tried removing the system from Sonosnet and moving to Wifi - that did’nt work either but you could try it….

Same here!!

Really hoping to get this fixed soon to actually enjoy the Dolby Atmos with my Arc.

I have fixed my issue, thanks to some help on this forum.

I removed my sub, then I was able to add the Era’s as surrounds, then added the sub back. Bingo

Thanks for the tip. Was thinking of doing that but nervous my sub wouldn't reconnect. I'll try it later today and hopefully I'll have the success you did.

@mikecollins99

This method worked! Surrounds connected and Sub reconnected. Now I only need to TruePlay. Thx again for the tip. 

...Well now the surround system won't TruPlay. When I get to the "begin tuning" section it won't work. The sub, Era 300's, and Arc are still connected but the app won't begin the TruePlay process. 

Found this researching the same problem. I just have an Arc and two Ones (no sub). I removed the two Ones from surround then plugged in the 300s. Their individual setup seemed to go fine and I got notifications from my network that they joined. Opened the app to add them as surrounds to the Arc but got the update required message, then no connection error. As others here I've rebooted router, Eras, and Arc to no effect. : /