I have a 12-inch, 1 GHz PowerBook running Mac OS X Tiger 10.4.1, and I had just installed iSync 2.1 yesterday when it was released by Apple. It was then that I noticed I hadn’t synced my PowerBook with the phone in a while, though the ‘Book had synced with .Mac.
When I attempted to sync the two devices, iSync told me it was unable to do so with the T616. I decided to remove it as a device, then re-add it. iSync picked up the phone during its device scan, but informed me it would be unable to sync with it.
I then turned to my other Mac, a 450 MHz Cube still running 10.3.9. I added the phone to the older version of iSync installed there, and it synchronized with no problem.
About half an hour later, I decided to revisit the PowerBook’s iSync version, and this time, the software recognized the phone, added it as a device, and synchronized with it. Since then, after making minor modifications to some contacts, I have made two more successful syncs with the T616. It would appear one simply needs to remove the device from iSync, wait a bit, then add it again.