Locked out of MS365 for Business admin account -> Action Required MFA loop

Originally asked on the Microsoft 365 forum but was redirected to this place. Pasting the original message below. I hope someone can help me out.

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Hi,

I am playing around with a MS 365 for Business trial licence and have setup mandatory MFA for all accounts (all accounts being two, controlled by me). Both accounts have been registered with the MS Authenticator and should be good to go.

However, the admin account is stuck in an "Action Required" loop asking for MFA setup. When following the wizard it immediately recognizes that MFA has been setup and sends me back to the login screen, which again prompts me with "Action Required". I'm not sure why it is not working.

Somehow I'm unable to insert screenshots, so I'll do it like this. First dialog contains the text:

Action Required

Your organisation requires additional security information. Follow the prompts to download and set up the Microsoft Authenticator app.

Use a different account Learn more about the Microsoft Authenticator app Next

When clicking Next, it shows the text:

Great job! You have successfully set up your security info. Choose "Done" to continue signing in.

Default sign-in method: Authenticator app or hardware token - code

Authenticator app Done

When clicking Done, I get back to the Action Required dialog. Rinse and repeat.

The second account is a user account (no admin rights, only global view rights) and is working perfectly fine.

What can I try?

EDIT: Forgot to mention that I've also done a password reset for the admin account, during which I had to enter a 2FA code generated by the MS Authenticator. So it is most definitely setup, but not being recognized during sign in.