The watchOS 5 beta seed, which went live on Apple's developer site following the WWDC 2018 keynote on January 4 has been temporarily halted. This is in order to fix an issue some developers might experience while updating. It's my understanding Apple wants to address the issues before resuming the seed.
If you go to developer.apple.com now, you'll see the following:
Seed 1 of watchOS 5 is temporarily unavailable. We are investigating an issue that may occur while updating. If you've experienced any issues, please contact AppleCare.
Betas for iOS 12, tvOS 12, and macOS Mojave remain available. The beta for watchOS 5 should resume soon.
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