Dear Andrea,
I faced a similar problem sometime back. I think the default XMII User role does not have sufficient authorizations to call Web service listener.
In my case, we have assigned an administrator role to the user and it works then. Try adding the admin role and try of it works. Then you can add the necessary authorizations or create a new role and assign it to the user and check again.
Hope this helps!
Regards,
Saumya Govil