Unable to invite Facebook friends to my app

Unable to invite Facebook friends to my app

I have a Facebook application which my users can invite their friends to
but when the recipient clicks on the 'Try Now' button to try my app, they
get the message:
Something went wrong. We're working on getting it fixed as soon as we can.
URL I am sending the inviting user to:
https://www.facebook.com/dialog/apprequests?app_id=[app id]&to=[recipient
UID]&messag
e=invitefriends&redirect_uri=http://[web server configured in
FB]/api/facebook/invitations/callback
And the inviter is redirected to the redirect and the recipient gets the
invite, they just can't click the button to try it.
Where is the 'Try Now' supposed to take the user?
One more thing - while this is an iPad application only, we are not yet
using the native Facebook iOS SDK - our login is done via the web
interface.
Second addition - our app has not been submitted for approval by Facebook
yet - I don't know if that might be related.