Trouble backing up to/from Win7 machines

This has been driving me crazy as well. I have two identical HP desktops (Windows 7/64). Networking options in Network and Sharing Center identical. Permissions on user files seem to be identical. On my Duo, one machine will back up on an archive bit basis fine, the other will only do so on a timestamp basis (test connections fail on this second machine in either archive bit or timestamp mode but will successfully back up in the latter). (When attempting archive bit backup with the second machine I get the "session setup failed: SUCCESS - 0" error.)
The first (working) machine is a new clean windows installation, the other about two years old. I would think I would have the perfect opportunity to pick up any difference in configuration causing the archive bit failure but no luck so far.