Here is a link to a video illustrating my problem. You'll see me trying to drag the scroll bars but the only thing that happens is sometimes they zip to the bottom and stay there. Also you'll see that when I hover over them the scroll bars illuminate and fill the entire height of the window - in the past they would only illuminate a portion of it, in order to show that more items were available above and/or below to scroll to. I also demonstrate that when I click on the icon of an item in the Fence and then scroll up or down using the arrow keys on the keyboard, it scrolls just fine through all the items in that Fence. Apologies if this doesn't work - have never recorded my desktop before...
http://gofile.me/71uKi/Cq06rB6n6
I've watched your video, unfortunately, I can not reproduce your issue on my 4K monitor even turning the scaling up to 300%, the scroll bars appear properly and it can be click and drag normally. Refer screenshot below.

I believe, it best to purge it and reinstall fresh, try these steps : [Link]. Just make sure to re-download the latest version from your account page. Reboot after the purge. Reinstall and reboot again after the installation. Retest and report back here.
Purge is a little different than those normal uninstall. It suppose to remove all traces of Fences from your system. So you can install again, fresh, as if Fences never installed before. Only your backup snapshots will be save.
Thank you,
Basj,
Stardock Community Assistant.