Do services behind a Sonicwall fail and timeout exactly at 15 minutes (900 seconds)?  This is usually due to a firewall rule that closes open TCP connections after a set “timeout”.  If you have a Sonicwall this is certainly the case as the default is 15 minutes.

Let’s take a look at your Sonicwall firewall rule (Advanced tab), namely the “TCP Connection Inactivity Timeout”:

See that “15″ minute value that it’s currently set to?  Change that to a larger number to allow your Sonicwall to keep the ports open during large transfers.