I had been using the GUI on Windows 10 without any issues from day 1 until it just stopped working the other day. Followed all tutorials, wikis and advice to no avail.
I found the solution by pure chance when I was clicking around in the network stuff.
Right-click on the Dwarf’s network adapter, choose Properties
On the networking tab click Configure
Go to the Advanced tab and make sure the correct value for the IP is filled in.
In my case, it was set to Not present. I have no idea how that came about. Changed it, and off I went again. Easy, if you know where to look…