Hello, @Kat D !
Why am I unable to sign in when attempting to use the sandbox?
There is a known issue that the sandbox team is trying to fix which is being tracked here:
If you are able to provide more detailed reproduction steps (verified permission, sandbox clock started, sandbox count changed, used a Hotmail email address, etc.) I'll add that to the report. This has been a challenge for the sandbox team but hopefully they'll be able to get a fix for this soon.
Screenshot of the what you would see if you encountered this error:
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