800 GB backup fails with "Access is Denied" or "Parameter is incorrect" error - QNAP timing out?

[outerheaven11]

QNAP unit: QNAP 653 in RAID 10

I am using Shadowprotect Server to backup my VM servers to this QNAP NAS. I have two VM servers, an older one and a newer one.

The first server has been in production for several months, no problems. It makes a backup every night to a remote share on the QNAP.

The second server is new and needs to back up about 800 GB of data in a full backup. When I run Shadowprotect for a full backup it gets partially through the process but eventually stops due to "Access Denied" or "The Parameter is incorrect" errors. I have double-checked my settings in Shadowprotect against my working server, and the credentials supplied are identical. In addition, the smaller System Reserved partition backs up with no problems, it's only the full backup file created from the 800GB C: partition that is having an issue. You can test the connection to the QNAP using Shadowprotect when supplying credentials and it passes the test. I am using the domain administrator name and password so I'm certain it has the proper credentials.

My working theory is that the QNAP is having some problem dealing with a file this size, and appears to time out the connection before the backup can finish. I have upgraded to the latest firmware, rebooted the QNAP, and verified connectivity and read/write access both through Windows and through the Shadowprotect program. Is there an issue with these units being unable to maintain open network connections for long periods of time during file copying? Any other reason this might be failing after having a backup file written to it continuously for several hours before failing?