Cheating, huh? Message apears when trying to select categories

So I have several custom post types, all with different taxonomies. The sidebar menu button under the custom post type was working smoothly until I changed the categories to non-hierarchical. It started showing me a blank page just displaying: “Cheating, huh?”.

Later I had to change it back to hierarchical and the error is still showing. There is only one admin so it shouldn’t be a permission error.

Any clues?