Hey, did you manage to resolve this issue? I'm experiencing the same problem on my iPhone 15 with iOS 18. TestFlight keeps showing the "Temporarily Unavailable" error, and I can't proceed further. I tried uninstalling, reinstalling, restarting the device, and signing in/out of my Apple ID, but nothing works. Since you mentioned it worked on another managed device, did you find out what caused the issue? Any help would be appreciated!