Tell us what you are trying to do?
I am trying to create a Custom Post Type with a slug that was previously used on another subfolder / separate WordPress install. That slug is /major. However, I have deleted that subfolder and the WordPress install on it but I still can't use that slug.
This one is quite odd as given that major isn't a reserved word then you should be able to use the slug. Have you tried to disable all the non-toolset plugins to see if you're able to create the slug when the plugins have been disabled.
If not can you allow me to have admin access to the site to grab a copy to test further here ?