we are facing the same problem. Maybe this data helps:
zend_extension = C:\xampp_7\php\ext\php_xdebug-2.5.0-7.0-vc14.dll
In Zend Studio preferences under PHP - Servers: One XAMPP Apache has been configured and its the only server. Break at first line is checked.
Base URL is http://localhost
and document root has been set on c:/xampp_7/htdocs (this might be different on your system)
In the tab "debugger" we have switched to XDebug on port 9000.
The debug session is being fired up through a generated bookmarklet from jetbrains on Mozilla Firefox browser.
Though its using a PHPSTORM IDE key Zend Studio accepts any incoming debug sessions. Sometimes its not working I have to say but after reading this post I fired it up again and no problem so far. Still have the feeling its a bit unreliable.