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The Puppet templates/labs-localsettings has
if ( array_key_exists( "SERVER_NAME", $_SERVER ) ) {
    $wgServer = "//" . $_SERVER["SERVER_NAME"];
}

Because $wgServer is not prefixed with http:, MediaWiki thinks https://servername links will work, thus several places in the code generate them, e.g. Special:UserLogin's _Use secure connection_ link and Echo notification e-mails. But most labs instances don't have working SSL, even via instance-proxy.wmflabs.org/

Project admins can't override this in orig/LocalSettings.php because these lines come after it is require_once'd.

Ideally the puppet LocalSettings would determine if https worked to the labs instance, and if not set
   $wgServer= 'http://' . $_SERVER['SERVER_NAME'];

Possible workarounds include unset ($_SERVER['REQUEST_TIME'] ),  overwriting $wgServer in some later config hook, and configuring SSL on the labs instance (bug 54065).