The url doesnt return the correct headers for php file_get_contents() to interpret. This could be due to the fact that when i go to that url it says i should provide a compaign_id.
So in your browser you get to see that but because in reality the headers send from that url return the status code 500 file_get_contents will interpret that as "internal server error". If you use chrome you can see this if you do ctrl+shift+i and go to the tab "network" then reload the page you will see "status" 500 internal server error. In firefox and opera you can also see this.
Missing parameter: _campaignID.