Whilst setting up the Plex service from the Sonos iOS Controller app I kept getting error messages after clicking the 'Existing Account' link (on the setup screen where you choose if you have a new or existing Plex account). Instead of showing the screen with the Plex pairing pin I saw a blank screen, then an error message after a timeout.
After a bit of trial and error I fixed the problem by disabling the iOS ad blocker Safari extension I'm running then restarting the Sonos Controller app and re-attempting the service setup process. Perhaps some hint could be added to the setup screens suggesting to the user to temporarily disable any ad blockers if the setup fails?