Fixed: sign-in is back, and two new checks before every release
At 0:53 JST on October 8 we shipped the fix. Signing in, opening worlds and talking to characters all work again. Your earlier saves are still there; reload the page and carry on.
To keep this from happening again, the release process now has two gates. First, a build must include the sign-in module and service addresses, or the build fails outright. Second, before a release goes out, an automatic sign-in check runs to confirm that signing in and refreshing an expired session both work.