Error code 500121

Hi @Jules,

Welcome to the Microsoft Q&A Forum.

The error code 500121 you're encountering typically indicates a failure in the Multi-Factor Authentication (MFA) process during sign-in to Microsoft services like Teams, Outlook, or Microsoft 365. This error often arises when the authentication request is not completed successfully, either due to a misconfigured Microsoft Authenticator app, expired credentials, or blocked access policies.

This matter falls outside the scope of what end users can typically resolve. To address it effectively, I recommend reaching out to your IT admin and asking them to take the following steps:

1. Use their administrator privileges to reset your Multi-Factor Authentication (MFA) settings. 

• Navigate to Microsoft 365 Admin Center, you can click Identity at the sidebar   

Microsoft 365 Admin Center website: Microsoft 365 admin center   

It will lead you to Microsoft Entra Admin Center (Microsoft Entra - Microsoft Entra admin center)   

Select Users    

Click All users     

Click on to the affected account name (your account)   

For example:   

• Select "Authentication Methods"    

• Click "Require re-register multifactor authentication"    

Next time the user sign in, the system will require them to set up a new authentication method. By using this, the affected user can re-set up the right authentication method      

1. Navigate to the Microsoft Entra Admin Center, and in the search bar, type “Block”. 
2. Go to Block/Unblock Users, and check if your account is listed. 
3. If your name appears, they should remove it from the block list, then try signing in again. 

This should help restore access and allow you to reconfigure your authentication method. 

Note: Please understand that our initial response does not always resolve the issue immediately. However, with your help and more detailed information, we can work together to find a solution. 

I hope this information is helpful. Please follow these steps and let me know if it works for you. If not, we can work together to resolve this. 

Thank you for your patience and your understanding. If you have any questions, please feel free to reach out. 

I'm looking forward to your reply.

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Hi @Jules,

Thank you for your response.

If the issue persists after following the initial steps, please ask your administrator to disable the Security Defaults setting to resolve error 500121.

To do this:

1. Log in to the Entra Admin Center.
2. In the Overview section, scroll down to the bottom.
3. Locate and select "Security Defaults."

Click into "Manage security defaults"

Change the status to Disabled, then click Save.

After making this change, please wait approximately 15 minutes, then try logging in again using incognito mode.

Note: If the affected user has any Conditional Access policies applied, please ensure they are disabled beforehand.

I hope this helps you with the error 500121. I'm happy to assist and truly hope the information shared has been useful to you. If you have any follow-up questions or need further assistance, feel free to reach out anytime. If you found this post helpful, please consider marking it as the accepted answer. Doing so helps others in the community with similar questions find solutions more easily.

Thank you for your thoughtful participation and contributions to the forum.

If the answer is helpful, click "Accept Answer" and vote positively. If you have more questions about this answer, click "Comment".

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