We sent the following instructions via email, and are posting them here as well for the benefit of others who may find them helpful:
Given the behavior you’re describing, we’ll want to investigate which antivirus/security/firewall programs are installed on the systems, and see if we can temporarily disable them, as they might be blocking the connection.
In the taskbar/notification area (down by the clock), there should be an icon for the security software. Once we’ve found the icon, we’ll want to right-click it, and see if there’s an option along the lines of “Disable Firewall” or “Disable Protection”. If given the option of how long to disable protection, choose “Until System Restart” so that it doesn’t turn itself back on in the middle of the transfer.
The steps are a bit different depending on which antivirus you are using, so if you’re unable to find a way to disable it, please email email@example.com and let us know the name of the antivirus software on each system and we’ll see if we can find any additional specific instructions to temporarily disable things.