After struggling a couple days, I finally figured out what was creating the following errors two Synology NAS boxes that backup each other through a sonic wall vpn.

Error Messages:

Error Network Backup 5/30/14 8:08 admin Shared Folder Sync failed to sync task [FolderSync 1] to [10.0.3.3] due to sync error.

Error Network Backup 5/30/14 7:47 SYSTEM Network Backup failed to backup task [Network Backup Set 1] to [10.0.3.3]. ([43] Connection to the destination server is timeout. Please check the following and try again:The destination server is connected to a stable network.Backup client and server is busy.)

After turning off jumbo frames as suggested here:

http://forum.synology.com/enu/viewtopic.php?f=160&t=76845

and then

Lengthening the Sonicwall TCP timeouts on the VPN tunnels as suggested here:

http://community.spiceworks.com/topic/143287-tcp-timeouts-site-to-site-vpn-sonic-wall

I finally came across this post that suggested that Sonicwall’s Intrusion Prevention System identifies rsync as a low level risk threat.

http://software.sonicwall.com/applications/app/index.asp?ev=appd&app_id=294&app_name=RSYNC

After disabling IPS on the LAN zones on both firewalls the backup ran as expected.