ProGet is not returning the
405 message, it's something on your network or server that you've configured to interfere/intercept web requests before they are sent to ProGet software.
Obviously I can't specifically tell you how to "un-configure" your server or network... but not enabling WebDAV would be a good start. It intercepts requests in this manner, as I mentioned in my earlier answer. If that doesn't do it, you'll need to keep investigating what is intercepting these requests.