Thank you for the quick assist, @chrisc!
After the fix on Plex’s end, I was able to link my Sonos account immediately. I can now see my Sonos speakers in the players list in the Plex mobile app.
It turns out this Plex account issue was also preventing me from using the Sonos app to access Plex. Earlier, I could add Plex as a music source in Sonos (successfully doing the authorization step in that case), but would immediately see the unhelpful error “Unable to browse music library” in the Sonos app when choosing Plex as a source. Being able to link my Plex account to my Sonos account in the Plex “Other Services” settings corrected that issue immediately.
