On a Verizon Pocket PC with Windows Mobile installed I was getting the error SYNC_5 when I was trying to sync up with the Exchange Server. This problem started after we changed the ISP service. All I did to resolve it was go into the AciveSync > Options > Server Tab and uncheck the calendar, contacts, and mail items. Close out of Active Sync then open again and re-check those items. Now Syncing works. Of course I had to resync everything again because by unchecking ActiveSync deletes all sync items.
I found my answer at: