Currently there is a problem with Facebook that affects FV Extender also. Basically you get an error message from Facebook when you try to accept a game request using Facebook directly or indirectly through FV Extender. This problem affects all games on Facebook.
The error message says: “Something went wrong. We’re working on getting it fixed as soon as we can.”.
So now it’s time to be patient and hope Facebook gets this fixed soon. I don’t have a workaround for this.
I hope you understand. Otherwise I’m all ears 😉