LoneWolf wrote:
GIMP Version: 2.8.22
Operating System: Windows
GIMP Experience: New User
Please help me to get my .xcf files to display as thumbnails. I have installed SageThumbs but they are black square. Can someone help, please.
It might be your Windows configuration (I don't have Windows), but on Linux all .xcf are displayed as thumbnails (GIMP 2.10.24 & 25), also the .xcf on external drive/dongle.

Or may be it's because you have installed this "SageThumbs" (for what/why? as your OS should generate thumbnails), which might be in conflict with your OS...
Or may be it's your GIMP config which does not put thumbnail when saving...
In all case you should not need an external software to see thumbnails in your folders/directories