The tree is not loading... Failed to load resource: the server responded with a status of 415 (Unsupported Media Type) Has anyone seen this issue before? Its not a major issue, I'm more interested in upgrading the DB at the moment but would like to see what's going on in the backend.

I have but its been a long time. Have you cleared all your localhost cookies or tried Incognito in your browser? Something in the back of my mind tells me that it's something to do with an existing auth cookie perhaps from another site you have logged in to on localhost.

Also, make sure any redirects are disabled, I think I've seen that when a post is redirected to a get (typically https/www/lowercase/trailing slash redirects which aren't excluding the CMS paths)