Internet Connection Sharing Service, port 1802, and Windows 10
Hello! I'm just posting this in the hopes that the information helps someone at some point and/or is searchable.
The Problem
The symptom in this case was that Fantasy Grounds would not open port 1802, but nothing else seemed to be listening on that port to occupy it. The way to tell if this is the specific problem (rather than a port forwarding or firewall problem) is with good ol' netstat or tcpview: Run Fantasy Grounds, load a campaign, then in a command prompt, run netstat -an | findstr "1802". If the result is not empty, this is not specifically your problem (although you could have a related problem, which is that some other server is taking the port, which I won't help diagnose here, check the sticky thread for that).
...and the Solution
If the command result is empty, then what may be happening is that the Internet Connection Sharing Service is reserving that port (and others). Use window button + r to bring up the run prompt, type "services.msc" into that. In the list, scroll down to find the Internet Connection Sharing (ICS) entry. Right-click it, choose "properties", then in the resulting window under the "startup type" dropdown, choose "disabled". Apply that setting, then in the same window, click the "Stop" button just under the startup type dropdown. Once you've done this, fantasy grounds should be able to bind to port 1802.
Unfortunately, anything that was depending on Internet Connection Sharing will now not work. So bewaaaare. You might have better luck running fantasy grounds with the -p option instead to select a different port.