Hi @Chr W Myrstad,
Thank you for posting your question in the Microsoft Q&A forum.
To better understand the situation and narrow down the cause, could you please clarify the following:
- When Microsoft Support requested the email address for the investigation, was it exactly the same email address that appears on the account currently signed in on the working PC?
- Are you still able to sign in with this Global Administrator account on the original PC where it is currently logged in?
- Since Microsoft Support mentioned they were unable to locate the account, could you please send the affected account's email address to me via private message so I can better understand the scenario? Please do not post it publicly.

The answers to these questions will help determine whether this is related to the account itself, the tenant configuration, or a device specific sign-in issue.
Thank you for your patience and understanding. I look forward to your update.
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