Sorry that i'm 4 months late in getting to you. I had the same exact problem with Multiplicity. Trial worked fine then the full version just stopped working. Oddly I got the full version working, as I was playing with some settings on my computer. I use multiplicity to connect between my and my wife's computer, transferring files and stuff, I never gave her access to my hard drives though (i.e. file sharing was off for my drives, but on for hers) I figured what the heck, on a shot in the dark i enabled file sharing on my drives and boom. Multiplicity was like hey hey! I'm gonna start working for you again.
Weird that the trial didn't have to have that enabled, and the full version does. Although in playing with Synergy I noticed that it seemed to work just fine the first time, and then gave me problems connecting. I don't know why that is, but for that program too, once I enabled file sharing it started working just fine as well.
HTH
-Shoeby