I've experienced similar problems backing up a Windows 2000 client (with the Cygwin Amanda client) from a Red Hat Enterprise Linux 2.1 AS (Itanium) box. The errors only happen when the Windows box is on the far side of a Fedora Core 2 bridge/gateway. When the Windows box is directly attached to the same network as the Amanda server, it works fine. Oddly enough, the Fedora Core 2 box that I back up in the same configuration also works fine.
Fortunatly, I have a plentiful supply of kernel hackers here, so if I get good tcpdump output(s) from tonight's run, maybe I can find out what's happening. -- JF