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On the login/join page, the notification and achievement dialogs close themselves immediately after opening. See the following screen capture:
(Safari 11.0.3 on macOS 10.13.3)
This was because you're currently in the Stack Overflow for Teams beta and your requests are routed slightly differently to allow access to the data secured within the teams that you are a member of.
It's now fixed! Thanks for the heads up :)