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.”.
UPDATE: How to report this specific problem to Facebook
You can report this problem to Facebook on this page. The more reports the more Facebook’s attention will get to this. Don’t mention FV Extender in the report. Just say that you can’t accept any requests from any games by using the App Center on Facebook:
https://www.facebook.com/help/contact/272649322838089