I could not log in to Area 51 in Chrome 62 under Linux. After entering my credentials and authenticating to my OpenID provider (Stack Exchange), I was redirected back to
https://area51.stackexchange.com/users/authenticate/?s=… and not logged in. (Full URL and error message omitted because it contains PII, available to staff in case there's something you can't find in your logs.)
I was already logged in on other SE sites. This turned out to be due to HTTPS Everywhere: after I disabled the extension and visited http://area51.stackexchange.com/, I was logged in automatically. Once I was logged in, I turned HTTPS Everywhere back on and the site worked.
Given that HTTPS support on Area 51 is supposed to be complete, there shouldn't have been any HTTP request at any point, therefore this is a bug. It had been previously reported, but at the time HTTPS support was not officially supported so it wasn't a bug.