I can't think of a reason why a web service would work in the editor, but fail in a built application (usually it's the other way around).
Does the "error out" terminal of the web service VI return any error once you've deployed your app? (If you don't have indicators on your front panel to see the error, you can use Debugging > Display Debug Assert - unbundle the error, and invert the error status for the 'assertion' input, and wire the status to 'message in', then you should get a message box at runtime if the web service VI is returning an error.)
I'm also curious what the "HTTP Get String" node is returning. To see this after building your app, you could do something similar with Display Debug Assert (leave 'assertion' unwired, and hook up 'response string' from the HTTP node to the assert node).
This problem is now solved but I don't know how... The HTTP Get String was returning the normal xml string.
I have an another problem though and I think you may help me. The problem is that after building my app, I can't refresh the items of my listbox by the properties.
There is an example "populating a listbox which works fine" on this website and the only difference between this listbox and my listbox is that the additional property option isn't set to access from diagram.
So my listbox returns a numeric value and the other returns just the items list.