… it MUST be the function files … same problems like you guys (i cloned a copy wp 3.9 with ducplicator plugin) and couldn’t log in … i change name of the “heavy modified” themes, i used (via ftp) and let wp switch to default theme “twenty fourteen” (!) – THEN i could log in, changed my themes to the original name back (via ftpo) and select the child theme via wp admin …then i also changed my passwort (admin) from within wp-admin and logout … no problems anymore, nor error message, normal loging procedure… hope that helps some of you …
