a little unknown tool called Graph Explorer can be used to troubleshoot Exchange online mailboxes. The tool is accessible at https://graphexplorer.cloudapp.net
Enter O365 tenant username and password. This will connect to your O365 tenant.
Once signed in will see below string in edit box:
Drop down and select users:
Append string with user with issues and select get to see diagnostic result
Scroll further down in the report to ProvisioningErrors section which should be blank as shown.
Example with errors below: