使用flowplayer（）。load来使用ajax响应作为flowplayer源

I want to load a url to video through a ajax-response, and use this url in flowplayer using the load-method. Simplified version of my script:

jQuery(".player").flowplayer({}).click(function(){

   var video_url = jQuery.ajax({ type: 'GET',       
                                 url: "loadurl.php",
                                 success: function(data) { return data;}
                               }).responseText;

flowplayer().load([{ mp4: video_url }]);

The error message in my browsers console: "HTTP "Content-Type" of "text/html" is not supported. Load of media resource XXX failed." (where XXX is the correct result, generated by my loadurl.php script). The URL is correct though, but the video windows shows "html5: video file not found".

Loading the video directly in the player, or using the playlist (as explained here) works perfect.

Any help?