This just started happening to me recently, trying to copy between 2 machines running Leopard. I get it when copying from the one I'm logged into, to the one I'm sharing, but I can successfully copy the other way around.
I can't pinpoint exactly when it happened, but I imagine it was the result of one of the point updates or security updates. Usually it's not a showstopper for me, since it's happening when both Macs are on the same desk, so I can just turn around and do the copy at the destination Mac. But it's really annoying. Especially since I have one folder that I copy between the Macs pretty frequently.
If anyone's heard anything about why this happens and/or how to fix it, pass it on!