Update:
I just disabled SMB and attempted the transfer via FTP instead. The transfer fails after about the same elapsed timeframe and the same problem occurs, though with a different error message (Operation Not Supported) - after the failure, I am able to connect to the host via FTP but am unable to open the share folder after getting about 5-10min into the transfer. Restarting the host clears the error and allows a normal connection and file transfer again, for about 10 minutes.
When attempting to open the share ACL settings within the web interface, the error reads "DirectoryIterator::__construct(/media/4058af09-918d-4153-a8c1-0e46d269728a/movies/): failed to open dir: Transport endpoint is not connected".