The page you are seeing is because of the AP trying to redirect HTTPS traffic to facebook. This is expected, and is what your browser is doing. If you mention that it worked before, could it be that another site was used than Facebook on HTTPS? Or that it was long ago?
Please check this blog for more explanation.
This also explains why you only see the error when using splash pages/captive portal redirects. It violates more or less the way HTTPS was designed.