Try using Panel on the Client to connect to the Server. Maybe if you have Gizmo, JRemote, or similar try that as well.
You could try turning off Authentication on the Server just to see if that changes things.
Otherwise, it is a bit weird it worked then just stopped.
If you go to "Left Navigation Bar > Services & Plug-ins > Media Network" on the Server, then select the "{MC Server name} (Library Server)" item in the server dropdown list, does the IP Address of the MC Client PC (the NUC) appear in the Activity Log as a Source? If it does, that would at least show that the Server sees the Client. You will need to know the IP Address of the NUC to check that.
The other thing you can try, if you know the IP Address of the MC Server, is to connect to the Server Library from the Client using the local IP Address of the Server, just to check connectivity bypassing the using Access Key.
So if your MC Server IP Address is 192.168.0.45, then rather than entering the Access Key on the MC Client (NUC) to connect to the Server Library, enter "192.168.0.45:52199" without the quotes, and the Client should connect.
If you want to check what JRiver knows about your IP Addresses, so you can check they are correct, in a browser enter the URL "
http://webplay.jriver.com/libraryserver/lookup?id=ABCdef", when ABCdef is your Access Key, and a page displaying your External (Internet) IP Address and your Internal LAN IP Address(es) will be shown. Your Router interface should show you what your actual IP Addresses are for the Server and Client. Or Google how to find that info.
See how that all goes.