Windows can be client but not server with OS X Sierra

[oneneptune 0]

If I set Windows 10 as the server, and OS X Sierra as the client, perpetual logs like:

[2018-09-27T10:45:12] NOTE: connecting to '172.21.7.91': 172.21.7.91:24800
[2018-09-27T10:45:27] WARNING: failed to connect to server: Timed out

A ping from each machine to other machine is successful. 

However - if I set OS X to act as the server, and Windows 10 to work as the client, it all connects and works instantly and smoothly.

What gives?