Here's another MDM enrollment failure issue we ran across recently that's caused by the CA not being available when the SCMDM Enrollment Service starts.  Fortunately the solution to this one is pretty easy:

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Issue: Enrollment fails and the following event is logged on the Enrollment Server:

Unknown error in Enrollment service:
System.ArgumentNullException: Value cannot be null.
Parameter name: data
   at Microsoft.Mobile.ManagementServices.EnrollmentServer.CryptoService.ComputeHmac(Byte[] data, Byte[] sessionKey)
   at Microsoft.Mobile.ManagementServices.EnrollmentServer.Authentication.AuthenticateServer(BootstrappingRequest rc)
   at Microsoft.Mobile.ManagementServices.EnrollmentServer.Authentication.Authenticate(RequestContext rc)

Cause: This can occur if the Certificate Authority (CA) was not running when the SCMDM Enrollment Service started.

Resolution: Restart the SCMDM Enrollment Service.

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J.C. Hornbeck | Manageability Knowledge Engineer