I'm sorry you haven't found error 64 in the forums as it is quite common and the reason why the network resilience
feature had been implemented in SQL Backup 6. Windows Networking implements some heavy, unreliable protocols and any copy operations are subject to the whims of network conditions.
Aside from upgrading, you can try increasing your session timeout.
http://www.red-gate.com/supportcenter/C ... 000212.htm
I also suspect, although never proven, that enabling NETBIOS over TCP on your network connection properties helps. I think this gets the connections using DNS instead of that ancient browser service
that Windows peer-to-peer networking uses.