Several times during during the last year, I have encountered a weird error with ADMT (or more precisely Password Export Server Service/PES), when trying to migrate users with their passwords from one forest to another (cross-forest user migration).
Something fails during install of PES on the source DC (mostly error with user privileges, but could also be other temporary issues), but the service installs. I then use the repair function from control panel, and everything seems fine.
When I try to migrate the user passwords, the error “RPC Server Unavailable” pops up. Firewalls are not the issue, nor are there firewall appliances between the client and server.
Solution: Re-install Password Export Server service on the source DC, as a repair just will not fix the issue.