PST Migrator (Import) from Windows 7 "The RPC Server is Unavailable"

EV 9 SP3 servers.  Windows 7 workstations.

We're having a blitz on PSTs at the moment.
The Helpdesk staff are trying to run some PST imports for users to their archives.
Rather than have them Helpdesk staff clog up the RDP connections to the EV servers, we want to have them run the PST Migrator (Import) tool from their own Helpdesk workstations.

I've given them the appropriate permissions through Authorization Managaer.

It all starts promisingly enough, but when they try to kick of the the import, they get the error:

"Migration of PSTs failed.
Check that the computer 'EVservername' is available.
Error: The RPC server is unavailable"

Any ideas?

This process works fine from my own PC, logged in to as me.

Thanks.