Outlook login loop + ServerError 401 across all devices
Has anyone dealt with this Outlook/Outlook.com authentication issue before?
Since Sunday night, my Outlook email has completely stopped working. I cannot send or receive emails, and I cannot access Outlook from anywhere. This is happening across multiple devices and browsers, including desktop webmail, the Outlook mobile app, and the Outlook desktop app.
To clarify, I CAN log into my Microsoft account itself. My Microsoft account still recognizes me, I can receive verification/security codes, and my Microsoft 365 subscription shows properly on my phone. The problem is specifically Outlook/email access.
After I enter my login credentials, Outlook either loops back to the sign-in screen or eventually shows a server error. One of the detailed errors showed:
ServerError: 401
Unexpected end of JSON input
BootResult: fail
It looks like authentication succeeds initially, but then the Outlook/mailbox session fails afterward.
I have already tried everything I can think of: password reset, clearing cache/cookies, private/incognito browsing, multiple browsers, multiple devices, different internet connections, reinstalling Outlook, removing and re-adding the account, and even had a tech remotely access my computer. Nothing fixed it.
The frustrating part is support. Outlook Mobile Support told me this appears to be an Outlook.com/account-side authentication or backend issue, not a mobile app problem, because it happens across all devices and browsers. But Microsoft support keeps telling me I need to contact Outlook support. I keep getting bounced around endlessly.
One agent gave me a link to a “form” and said it would reset my authentication, but it was literally just a password reset form, which I have already done multiple times.
Has anyone had this exact issue with Outlook.com where the Microsoft account works, verification codes work, but Outlook webmail fails after login with ServerError 401 / BootResult fail? If so, how did you get it escalated to the right team?