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.