http://facebooksdk.net/docs/phone/controls/login-ui-control/

"Given URL is not allowed by the Application configuration.:
One or more of the given URLs is not allowed by the App's settings.
It must match the Website URL or Canvas URL, or the domain must be a
subdomain of one of the App's domains."

There is currently a bug which will prevent Facebook Login for Windows Phone
from working if you have any entries in
the "Valid OAuth redirect URIs" field in the Advanced section of your app settings.
This can be worked around by adding "https://m.facebook.com/dialog/return/ms"
in this field.