Error 500 logging into ADO or Private Nuget feed

Try to fix by the following:

1. Verify Guest Account Permissions

• Ensure the guest account has the correct permissions in the ADO organization. Check the user's role and access level in the ADO subscription. They should have at least "Basic" access or the appropriate role for accessing private feeds.

2. Check Azure AD Configuration

• Confirm that the guest account is properly configured in Azure AD. Sometimes, issues arise if the guest account is not fully synchronized or lacks necessary permissions in the Azure AD tenant.

3. Private Feed Access

• For the private NuGet feed, ensure the guest account has been granted access to the feed.

4. Authentication Methods

• The error TF400813 indicates an authorization issue. Ensure the guest account is using the correct authentication method (e.g., Personal Access Token (PAT) or OAuth). 

5. Clear Cache and Retry

• Ask the user to clear their browser cache and cookies, as cached data can sometimes cause login issues.
• If using a different machine still results in the same error, it confirms the issue is account-related rather than device-specific.

6. Activity Logs

• Check the activity logs in Azure AD and ADO to identify any failed authentication attempts or permission-related errors. This can provide more insight into the root cause.