I just noticed that trying to visit one of the per-site metas using HTTPS will (after confirming the security exception due to the limitations of SSL wildcard certificates) give a 403 error:
Yes, I know that HTTPS is not officially supported on SE (yet). But the per-site metas used to work over HTTPS, so this is a regression.
I did some extra testing, and it appears that this error shows up only for the per-site metas with a four-part hostname of the form
meta.sitename.stackexchange.com. That is, the following meta sites with custom domains work fine over HTTPS:
but any other per-site metas, (like, say, https://meta.math.stackexchange.com) are broken.
From the error message, I suspect this regression has something to do with the use of the CloudFlare CDN. Perhaps something needs to be (re)configured in SE's CloudFlare settings to make this work again?