Thanks for the feedback. I am experiencing this issue via the iOS web app also. I can see my Sonos speakers within the app, I select one and I just get a spinning wheel. Whereas I fire up Spotify and cast to Sonos with no problem via the same speaker selection process.
As I was exploring this, I found a workaround. I selected my iPhone from ‘choose a player’ and then hit the Airplay button next to it, from there I can select my Sonos speaker locations. If I go back and try and select one of my Sonos speakers directly from the player selection within Plex, I get the same response, a spinning wheel and Plex moving through the album titles without connecting or playing.
But I’ll take a workaround for now!!
Thanks
Kristian