Elgg API

I am trying to use the elgg api.

When I call the method in a browser directly, everything is fine
(http://server.de/services/api/rest/xml/?method=ag_makeVideo&user=186&w=b074f9fb61f1a8e9874c330c31564aea&ytid=PLpFNVZEo5I&duration=4&title=Cover&description=Mit+Text)

 

but when I try to call this method in a script from another Server using file_get_contents, something like that returns:

<?xml version='1.0' encoding='UTF-8'?>
<elgg>

<status type="integer">
-1</status>
<message type="string">
Unable to save new object's base entity information!</message>
</elgg>

What's wrong here?