How can plugin built feed be transformed into a real and working RSS feed?

I want to bring the "Global Author Posts Feed" Adress to the people, the problem ist, that the URL is a PHP Adress. The built-in RSS Reader of Wordpress can read it, but "outside" I have not found any reader, who can read this adress as a real RSS Feed and I always get a 404 Failure back!

On my site it looks this way:

How can I force the Plugin to transform the feed into a real and working RSS Feed? Perhaps an update by WPMU Dev would be very useful!

Has anyone else this problem and found a solution?