I just added the Category Widget to one of my layouts. Selecting a category from the pull down menu directs to 404-Fancy meeting you here!
Here is a URL to the site: http://artandactivism.org/category/blog/
I think the issue is that the widget directs to for example: http://artandactivism.org/blog/category/blog/art-and-class/
Note the additional "/blog" directory before "/category" in the current widget.
Suggestions for fixes welcome! Thank you!