Failed to add a custom field in settings
You're getting that error message because your PHP environment is displaying PHP Strict mode error messages.

By default, Craft will explicitly disable those if devMode is enabled (error_reporting(E_ALL &amp; ~E_STRICT);) and disable all messages if devMode is disabled (error_reporting(0);).

So my guess is you've either got a plugin that's messing with error_reporting and re-enabling strict messages or you've doing it from your public index.php or craft/config files.

Note that in the upcoming Craft 3, we've made it so that you can run Craft with PHP strict error messaging enabled.