Also a "Repair" on Office
Indications from other threads are that the error "ActiActiveSync encountered a problem on the desktop" is pointing to the Office "APIs".
After trying other things from various threads (including what's mentioned on this thread), what worked for me was to do a "Repair" on "Office 2010".
OS: Windows 7 Ultimate (64 bit)
Windows Phone 6.1 (Sprint Mogul)