Bumping this old thread, because we're seeing the same problems.
We're running 64-bit 7.1.4 Network Edition on RHEL5, and we've got Mac OS X clients with the iSync connector (7.1.3201). I've tried trashing the user's Zimbra folder in ~/Library/Application\ Support, but that hasn't helped.
If I modify the username, it allows me to sync... for a while. Eventually, it stops.
This started happening once we upgraded the server from running 6.0.10 on Mac OS 10.5.