You don’t need to point the Docs plugin to the category - this plugin replaces the Docs plugin. That might be what is affecting things for you, as it would cause a conflict with one overriding the other.
All you need to do is make sure doc categories enabled is checked, and then create the index topic in the category, adding it to the category’s settings in the Index topic field. If you still have the category set in the old Docs plugin settings, then you should remove it from there.