Check the log from the MobileAsset Windows Service. If there's no related error, there may be something wrong with the database file on the device. Copy the Database folder from the device to the PC as a backup, then wipe out the device and set it up again. With the newly-downloaded database, try a full sync. If this succeeds, quit MobileAsset on the device, copy the old database back to the device, then try the sync again. If this fails again, there is something wrong with the device's database. In that case, we'll need a PC database backup, as well as the device's Database folder (all files) to investigate.