When seo admin permalink settting "Strip the category base (usually /category/) from the category URL." is set and there's a static slug in the general WP settings such as /blog/%post-name% the plugin will remove the static slug when it shouldn't.
So instead of .com/blog/categoryname/
you get this .com/categoryname/ which essentially translates to a 404
Need to get that fixed. Thx Joost