As reported by @AlexisJazz (see T309310), our basic configuration file editor is failing to save changes because it opens https://meta.wikimedia.org/?action=submit instead of https://meta.wikimedia.org/w/index.php?action=submit.

Although this was working before (something must have recently changed in Wikimedia servers), it makes sense to change to https://meta.wikimedia.org/w/index.php?action=submit.