That's due to the recent release of the official 0.5.1 version which supercedes the release candidate versions and rendered them unavailable on Build.
Normally you'd get an auto-update, but you'd need to select 0.5.1 as target version.
@polystyrene, how long did you leave the devices sit in Safe Mode?
Normally Safe-Mode-Healer should get your Photons out of that once the cloud finds your device stuck in SM.