I restarted the windows PC running plex server and my iPhone, but still when I cast i get the same thing saying that I need a plex pass to cast to Sonos.
Are both your plex server and the plex app on your phone signed into the plex.tv account which does have the Plex Pass?
Sometimes users have created several accounts
Also keep in mind that switching off the whole server machine and back on is not the same as stopping the Plex server and restarting it.
I figured that a full restart of the windows computer would include restarting the plex server and more.
Anyway, i have since restarted the plex server to no avail.
I downloaded and logged in to Plexamp and it worked fine.
So I assume I needed to re-authenticate my iOS device to get it working.
I jumped back to plex app to do that and… it was working too.
Thanks for your help and suggestions.
Hopefully this will help someone else out in the future.