I can't update settings for certain custom post types (on my site, these have the slugs maintenance and emergency-sops)
Error message when trying to save is: You cannot use this slug because it is already used or a reserved word. Please choose a different slug.
All I am trying to do is turn on the "Author" field. I've just successfully done this for a number of other post types, and it worked fine. I've tried deactivating all plugins bar Toolset and reverting back to 2019 theme with no success.