Depends on the source you’re listening to. Some sources only allow a single stream at a time, others have ‘family’ style accounts that allow you to have multiple streams. Sonos is only enforcing what the streaming company says, it isn’t up to Sonos. 

Not sure if this is the point exactly. I see complaints here of people having different households on one account. They seem to have problems since May because the new app seems to handle households differently. Could that be the problem here?

Doesn’t matter whether it is one Sonos account used in multiple households, or multiple Sonos accounts. The simple fact is that if you’re using a single ‘account’ from a streaming company, they’ll often ‘block’ multiple streams, whether they’re on Sonos or not. You’d get the same behavior if one originating device was a Sonos device, and the other an Amazon device. The streaming company determines the number of allowable simultaneous streams, Sonos merely does what they’re told. 

What is the source for the music? A streaming service such as Spotify, Sonos Radio or independent local libraries in each household? 

Thanks, it is likely a Spotify issue.