I've never tried QuickSynergy before - I just use the command line version from sourceforge.

From the sound of your setup you're running the PC as the server and the MBP as the client? That's the reverse of mine (I have my MBP as my main machine), so I've not tried it that way round either. However it should work no problem.

My suspicion would be that the PC is firewalling the Synergy server - so the MBP tries to connect and then just dies after a while through being rejected.