I had this problem as well, but it was the name of a taxonomy. I was unable to save it (description, custom fields) until I changed the slug. Once I did I was unable to change it back.
After searching a finding this post, I searched the media library, and sure enough there was an image with the title matching the exactly slug of the taxonomy. Even after I changed the image title (and refreshed permalinks) I was still unable to change back the taxonomy name though.
Thank you for this post, it saved me a lot of trouble!
I really hope this is prevented in future versions of WordPress! When you have thousands of images it’s difficult to make sure they don’t have the same name as posts or taxonomies, and there just shouldn’t be conflicts anyway.
