Remote registry backup

Here is the problem.

One NT 4.0 SP6a Server installed as BDC runs Exchange servers. Daily backups are performed from this server using ArcServe. The following things are backed up:

1. This server completely including all drives, Exchange and registry.
2. PDC and another BDC completely including drives and registry.
3. Member server from other domain completely (drives and registry).

The problem is that registry from PDC and another BDC are NOT backed up with error
DRIVE=\\servername\admin$ – no such logon session
But local registry and registry from member server are backed up OK.

Can’t get what’s wrong.
Any ideas are greatly appretiated.