I came into problem recently. After upgrade of Exchange 2013 CU2 to CU3 my ECP stopped to work. I tried to:
- re-create ECP virtual directory
- renew certificate (which expired recently)
none of those helped.
Then I checked authentication methods for my virtual directories and found, that Exchange 2013 CU3 setup messed up my OWA authentication methods. The goal here is to have both virtual directories set with the same authentication methods. In my case FBA and Basic.
Set correct authentication methods and run IISreset on problematic backend exchange servers.
Get-OwaVirtualDirectory | Set-OwaVirtualDirectory -FormsAuthentication $true -BasicAuthentication $true