Same here: the app prompted me to reconnect the Plex account. I deleted the old Plex service in Sonos and readded it. Then I was redirected to the Plex page where I found the security alert message (188.8.131.52 Dublin, Leinster, Ireland). Once I linked the account everithing worked again.
For the ones that after the account re-autentication encounter the "unable to browse music" message: are you sure you used the right credentials to access your Plex account? For example, if your account was opened with a @yahoo.com email and during the re-autentication you automatically sing in with your @gmail.com google account... You'll end up with a new (and empty) Plex account linked to Sonos, and get the "unable to browse music" message!