a new MS authenticator installation is requiring to sign in for an existing account

Hi @Remco Nijkamp, 

Thank you for posting on Microsoft Q&A forum and I'm sorry to hear you're having trouble logging into your Microsoft account.    

This issue commonly occurs when your Multi-Factor Authentication (MFA) settings need to be reset, particularly after changing devices or reinstalling the Microsoft Authenticator app. Since your new device hasn’t been configured with the Authenticator yet, you're currently unable to access your account. The app remains linked to your previous device, which is why you're not receiving the authentication codes.  

If you are the only Global Administrator on the tenant, please follow the steps outlined below to proceed with account recovery and regain access.  

Option 1/ Reach out Microsoft Data Protection Support by phone (Primary Method)    

To regain access to your admin account as you can't access the Admin Portal, you can try reaching out to our Global Customer Service phone to raise a request for resetting your authentication method here: Customer service phone numbers - Microsoft Support. During the call, request to speak to an agent, and share with them every detail related to your query and also mention that you are the only admin lost access to your account. This should allow you to contact the appropriate team so you can solve this incident as soon as possible.   

Here are some tips and an example of a prompt to help you navigate the IVR more effectively:         

In some countries, it is an automated conversation like:   

IVR: What kind of problem are you concerned about?         

You: Authenticator.         

IVR: What kind of product do you use?         

You: Office 365 for business.         

IVR confirmation: education or company account?         

You: For companies         

IVR: Are you an administrator?         

You: Yes.         

IVR: Do you have another administrator in your organization?         

You: No.     

IVR: Do you need a... Service request?           

You: Yes. I need to create a ticket. Please send me direct to the Data Protection Team.    

Option 2/ Create a temporary account (trial tenant) to submit a support ticket (Alternative Method)   

If you still cannot reach to a live agent, there is still a workaround, you might consider registering for a new tenant by signing up for a trial subscription via this link Microsoft 365 Business Plans and Pricing | Microsoft 365. This would allow you to create a new tenant following the prompts provided. Once sets up, you can access the admin console of the new tenant and submit a support ticket requesting to speak with the Data Protection team on behalf of your previous tenant.         

• If there is someone else with admin access, you can reach out to your IT administrator and share the steps outlined in the article Manage user authentication methods for Microsoft Entra multifactor authentication. This will help them assist you in resetting your MFA settings and restoring access to your account.   

I hope this helps you regain access to your account quickly. I'm glad to assist and truly hope the information provided has been useful. Please feel free to reach out anytime if you need further assistance.   

If you find my post helpful, kindly consider marking it as the accepted answer. Doing so can assist others in the community who may have similar questions in finding solutions more quickly.   

Thank you for your kindness and contributions to the forum. 

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".   

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.