I have a category with some posts in it. I had previously protected this category by putting it in a Negative Rule for visitors access level.
I have since removed the rules; there's no protecting rules at all. Yet, trying to go to a post in that category generates 404.
I have changed the permalink to date, numeric, default etc. and they all work. I use, and was at the time of post creation, using /%category%/%postname%/ for the permalik structure. This doesn't work.
Is it possible that membership had anything to do with it?