I had an empty subcategory that kept getting the error message to make sure it was empty. I had removed all the products to a different category, and waited weeks, and still kept getting the error message. Here's what finally worked.
I copied and pasted the category link to a new browser without being logged in. Lo and behold, a couple of the products that I had moved, and were showing in the correct new category in my back end, were still showing in that old category on the public view! I clicked those products individually, changed the category (again!) under "Edit Details," and after a day or so I was able to delete that subcategory with no problems!
