I've had the EXACT same problem you described for several months and just fixed it!
I use a 64 bit Dell PC as my primary computer and host/server, running Synergy's x86 (32bit) version 1.5.0. (In troubleshooting, I replaced the 64 bit version with the x86 version. Both the 64 and 32 bit versions would connect with the Synergy client on my old PC running XP, so I didn't bother changing back to the x64 version.)
The Mac/client operating system is 10.4.11. Synergy version 1.4.8 used to work with the PC running XP; however, it wouldn't work after I started using the 64 bit Dell as the server. Also, when I tried going back to using the XP PC as the server again, it had the same problem. Based on that, I assumed that there was some problem on the Mac.
None of the old or new Mac versions on Synergy's website worked. I found a really old version called SynergyKM, ver. 1.3.0 (copywright from 2002!) on another old Mac and it worked. NOTE: Before installing it I used Finder to search for all old synergy files and deleted them - this was probably a key step!
I don't know where the old version that worked for me can be located online. It's one of the GUI versions that came as a .dmg file - not like the pre 1.4.5 versions on Synergy's site that come in .zip files. If you'd like me to send it, send an email (my name shown on this post at gmail.com).