This is just a quick post to share with others a potential solution to your network scanning issues with the Brother MFC-J6920DW printer/scanner.
I had recently bought the device because I wanted a larger format printer, along with the ability to scan and copy. It’s a very nice unit, and has so far worked well as a printer, but I ran into some issues when trying to scan across the network with it. I was constantly saying “An error occurred while communicating with the scanner. (-9923)”. This is under OS X 10.11.3 (El Capitan), just for reference.