Not sure why you need to install older version instead installing the latest V4.22? Also, please post a full desktop screenshot so I can see and understand the issue better.
Thank you,
Basj,
Stardock Community Assistant
Otherwise I couldn't get Fences to open for some reason but as said the reinstall didn't improve things anyhow.
As for screeenshots, here are two that show the issue, Folder Portal before a display was enabled or disabled:

And here after a monitor was enabled or disabled or any navigation inside the Portal was done:

In my extensive testing now I noticed it doesn't happen 100% of the time but in 8 of 10 tries enabling and disabling a monitor the background would change to black and it required a restart of explorer.exe to restore to normal. The high transparency is just for illustrative purposes. Note, this exclusively happens to Folder Portals, normal Fences are fine.
Also sorry for the weird colors, HDR screenshots aren't easy to take.