Matt,
Based on my own experience when running ACT! locally on a Win2k machine, but grabbing data that was on a ESSG 4.1.2 server - it is an issue with Samba and file locking/access across the network.
The version of Samba in 4.1.2 is like 2.0.7 I think...and you need 2.2.1a or later. I'd recommend that you run an upgrade to the latest SME v5.1.2 and see if that fixes your problems. My bet - it will.
Let us know, and good luck.
Regards,
Patrick