Job Failed - An unknown error occurred within the NDMP subsystem

I keep getting an error when backing up my Exchange 2007 server.  I have searched the knowledge base but there is nothing for the error message

" [serverbackupjobname] -- The job failed with the following error: An unknown error occurred within the NDMP subsystem."

That is all it says, this is not a VM or Hyper-V server nor a Netware or Red Hat server.  And what is NDMP subsystem??

Thanks

Cathy