When we see an error like this:
Phabricator thinks you are using HTTP, but your client is conviced that it is using HTTPS. This is a serious misconfiguration with subtle, but significant, consequences. See Documentation
The docs advise to use
$_SERVER['HTTPS'] = true;.
This is incorrect in the situations where Nginx is used with PHP-FPM. Instead one should use
fastcgi_param HTTPS on; in the Nginx configuration.
This seems like a documentation bug. We should not blatantly tell PHP that HTTPS is on all the time. We should have Nginx, which is fronting the SSL certificate and https connection, tell PHP when HTTPS is in use.