No users can login to either the ASP Admin or End-User portals

Problem

No users can login to the ASP Admin or End-User portals.  

Error Message

The following error is seen after the login attempt: 

Internal Server Error
Access Denied
You (john.smith@contoso.com) don't have permissions to access the requested feature.  

Cause

There can be various reasons for all users not being able to access the portals.   

1. The default domain in Azure Active Directory was changed, before the next daily Directory Sync can be run to update all the user SMTP addresses. 
    
2. The Client Secret of the Azure Active Directory Sync application expired. 

Solution

Create a Support case in order to resolve this issue.