I havent tested the remote API in SVN trunk for some time, so there might be a bug.
It looks as if the soap header is send and the script stops then, maybe there is a php error where the error message is surpressed by php.
Try to edit the file /usr/local/ispconfig/interface/web/remote/index.php and add this:
ini_set('error_reporting', E_ALL & ~E_NOTICE);
right after the inclide of the app.inc.php file