Scenario: If your receive the following error when using Hybrid Modern Authentication for Exchange OnPremises, run the following scriptlet below. Our problem was a misconfigured URL set on the get-clientaccessserver where it was pointing to a Servername, and not our shared namespace.
Scriptlet:
Get-MapiVirtualDirectory | FL server,*url*
Get-WebServicesVirtualDirectory | FL server,*url*
Get-ClientAccessServer | fl Name, AutodiscoverServiceInternalUri
Get-OABVirtualDirectory | FL server,*url*
Get-AutodiscoverVirtualDirectory | FL server,*url*
Get-OutlookAnywhere | FL server,*url*
Ex-Shell 