Same issue here.
It just stopped working from today.
I retrieve the access token from azure https://login.microsoftonline.com/cloudesireeng.onmicrosoft.com/oauth2/v2.0/token client_info=1 grant_type=client_credentials scope="openid profile offline_access https://graph.microsoft.com/.default" client_secret="SECRET" client_id=CLIENT_ID Content-Type:"application/x-www-form-urlencoded; charset=UTF-8"
and the I use the Bearer to retrieve the customers from partner center, it worked until yesterday, from today is says: 401 (invalid_grant)