I was able to get v5.0-5 working with vbox 5.1 by commenting out the version check line and replacing it with a static value in the endpoints/api.php file:

// $response['data']['responseData']['phpvboxver'] = @constant('PHPVBOX_VER');
 $response['data']['responseData']['phpvboxver'] = "5.1-0";

Src: https://sourceforge.net/p/phpvirtualbox/discussion/general/thread/565b7f31/