If IIS 6 throws a 403 error, it could well be because you are trying to use a PUT or DELETE command from our API, and these are not allowed by your local install of IIS. The API call is blocked by IIS before it even gets to SpiraTeam/Test/Plan.

To fix it, you need to add the relevant verbs to IIS. The top answer on this Stack Overflow question explains what to do (note the exact route to edit the relevant service may vary depending on which version of Windows the server is running on).