Are you using any other login-related plugins, such as a "log in with facebook" type?
Or possibly another security plugin that has a "hide PHP version" option?
Mededot: Is this site on the same host as your others that are running Wordfence without any problems? Wordfence has a comment near authActionNew() about working around an issue prior to PHP 5.4.0, but if it's on the same server, that probably wouldn't be the problem (unless individual accounts can be assigned to a certain version of PHP in the reseller software?)