I've been seeing this problem more freqeuently lately.
When I move the mouse from the primary, Vista x64, to the secondary, XPSP2, the left-mouse does not work...it either does nothing or acts as a right-click. Moving back to the primary the mouse continues to function correctly.
Using the mouse on the secondary and left-clicking restores proper functionality. I have added the app to the Windows firewalls on both computers.
Multiplicity is on a secondary subnet with static IPs.