As OP I've been using Fences for years. Though, I just cannot create a new fence.
Re-installing with admin rights doesn't work for me.
Though I'm not sure when it stopped working, I've purchased this computer in August and installed Fences and created a couple, now I can't.
Can it have something to do with the latest Windows Update?
If I find the correct info I'm on Windows 10 Pro, version 1903, build (I guess) 18362.356
The only recent update is one for .NET (KB4514359) and a cumulative (KB4515384).