Microsoft 365 Admin Account MFA Error (Error Code 500121) – Unable to Sign In

Question

I'm trying to sign in to my Microsoft 365 business account (******@XXXX.onmicrosoft.com) as the administrator, but I’m unable to complete the MFA (Multi-Factor Authentication) process. The Microsoft Authenticator app does not receive push notifications, and no OTP code is displayed.

This issue started after I reset my smartphone. This account is the only administrator account for the tenant.

When I attempt to log in, I receive the following error:

• Error Code: 500121
• Request ID: XXXX-XXXX-XXX-XXX-XXXXXX
• Timestamp: 2025-07-29T00:05:50Z

Currently, there are no other authentication methods (such as phone number or email) registered, so I’m unable to reset MFA on my own.

I tried contacting Microsoft Support, but phone support is not available in Korea, and I can only reach out via email (XXXXX@microsoft.com).

How can I reset or recover MFA in this situation?

Any help would be greatly appreciated.

Answer 1

Dear @경헌 조,

Thank you so much for contacting Microsoft Q&A Forum. 

Please understand that our forum is a public platform, and we will modify your question to cover your organization email address in the description. Please notice to hide these personal or organization information next time you post error or some information to protect personal data. 

Based on your description, I understand that you are being locked out of a Microsoft 365 admin account due to MFA failure after a phone reset. Also, I appreciate that you have tried to fix it by alternative methods, but it did not work. So, you have to reset your MFA in this situation. You also mentioned that you are the only Global Administrator and you are completely locked out from the tenant. You might need to contact us via phone service number: Customer service phone numbers - Microsoft Support. The agent will be able to create a ticket for you under the affected tenant and then transfer this ticket to Data Protection team who could help you to regain access. 

Please note that this team is always overwhelmed, so you might need to wait for them to call you, this should take 3-10 working days. I know that this is not a convenient solution to hear but it seems to be the only way, and it's recommended that you should have a secondary/backup Global Administrator to fix this issue on your own. 

In case that the phone line cannot reach and you only have one admin account so you cannot raise a support ticket by current account, I suggest that you can initiate a support request by starting a free trial here (using other admin account to raise support ticket): Microsoft 365 Business Plans and Pricing | Microsoft 365 

Assuming that you have already created an account after registering a free trial and you can go to Microsoft 365 admin center and click "Help & support" icon to raise your ticket about resetting MFA of the old admin account. 

You can follow this link for more instructions: Get support - Microsoft 365 admin | Microsoft Learn

I hope this information can help you to reset it so that your work can be back on track. 

Wish you all the best!

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