Authentication with Office 365 API problem

Hi there,

I had the connection between Post SMTP and Microsoft365 API working perfectly fine and then all of a sudden all email sending is failing.

I did get this error message:
Code: 401, Message: Unauthorized, Body: {“error”:{“code”:”InvalidAuthenticationToken”,”message”:”ArgumentNull”,”innerError”:{“date”:”2026-09-10T12:09:56″,”request-id”:”2e5a93b5-3341-4d0c-b6cf-15155931b040″,”client-request-id”:”2e5a93b5-3341-4d0c-b6cf-15155931b040″}}}

So, I have created multiple times now a new app in azure and double checked that all is created according to the step by step guide of post smtp, but I always do get the same error code.

Now even when I want ro rerun the configuration wizard, I get this error:
Error retrieving access token. Try Again Now.

I have wordfence installed, but I did try to run the firewall in learning mode, disable the firewall and even deactivate wordfence completely, but still the same problem.

Does anyone have an idea how I can fix this problem?