User attempting to access Microsoft Dynamics CRM Server 2016 receives a: “Server Error 404 - File or directory not found.” web page

Problem

You’ve recently deployed a new Microsoft Dynamics CRM Server 2016 environment:

… and had accounts created for the users in the organization but noticed that some users receive the following web page when attempting to access the portal via IFD and ADFS:

Server Error

404 - File or directory not found.

The resource you are looking for might have been removed, had its name changed, or is temporarily unavailable.

You’ve confirmed that the user is enabled in CRM:

You proceed to navigate to the ADFS sign page that the CRM deployment uses and sign in as the user:

What you quickly notice is that the following error is displayed when you sign in with the user’s credentials:

No Microsoft Dynamics CRM user exists with the specified domain name and user ID

A Microsoft Dynamics CRM user record does not exist with the specified domain name and user ID.

Solution

Note that I am by no means an expert with CRM but I made a “best effort” attempt at troubleshooting the issue and was able to identify the following:

1. The user account appears to be valid in the CRM portal because it pulled the correct information for the user
2. Although the account is displayed in the CRM portal, it appears it is not tied to a CRM record, which appears to suggest the Active Directory identity does not map to a CRM identity
3. Disabling and re-enabling the account does not correct the issue

What ended up enabling me to correct this problem was to:

1. Create a new temporary Active Directory user, which would not be created in the CRM directory (I could not use an existing user because they would already be created in CRM)
2. Quickly change the User Name field of the user experiencing the issue to the temporary user account
3. Save and Close the account properties
4. Change the User Name field of the user experiencing the issue from the temporary user account back to their identity
5. Save and Close the account properties

This appears to force CRM to map the user’s Active Directory identity to a CRM user record.  Below are a few screenshots that show the process:

Once the above has been saved and closed, reopen the account properties and change the User Name back to the correct identity.