Facebook Apps stopped working - Bad request in MiniFB.oauth_access_token

[Julian]

As of the last couple of days both my apps (development/production)
stopped working. I'm not sure if this has something to do with the
migration to HTTPS.

After I go through the Auth dialog I get back the ?code= and then make
a request to get the access token using:

MiniFB.oauth_access_token(KEY, REDIRECT_URI, SECRET, params[:code])

The URL it requests is

https://graph.facebook.com/oauth/access_token?client_id=KEY&redirect_uri=REDIRECT_URI&client_secret=SECRET&code=CODE

This used to work perfectly but now it's giving a 400 Bad Request,
"Error validating verification code".

However, if I make a request to this URL in my browser (logged into
Facebook) it works, giving back the access token.

Has anyone else noticed anything like this?