Hmmm, looks like Facebook tightened their security standards. If you are using HTTPS (secure browsing) it no longer seems to work... ever. If you drop the S from the HTTP it will work, but if you have secure browsing turned on in Facebook, it will just re-add the HTTPS and you'll be back in the same boat again.
I suspect this is because Facebook is no longer liking the untrusted certificate I used on our server. I'll check into it to be sure this is truly the problem but it does look that way. A 'trusted' cert generally costs about $99/year so unless there is a hue and cry to retain secure browsing, I suggest users turn it off while playing.
Wish I had a better answer, especially as I do security work in my day job!