Hi John, hi Sylvia,
I finally managed to get it work.
My workaround was changing the redirect_uri manually in the address bar, from https%3A%2F%2Fapps.facebook.com%2F319872171403936%2F to https://www.frankentipps.de/blog/ in our case (if I remember right, I was just trying around and suddenly it worked).
Then I could grant all the required permissions – and it worked.
I hope this helps,
Best regards
Matthias