On the Wordfence options page, try turning on “Disable config caching” near the bottom of the page, and saving the options.
If your site is somewhat busy at the moment, the configCache.php file is probably being modified while trying to update the plugin, which can cause that warning from WordPress.
In some cases, you might have to remove the configCache.php file manually by FTP, or just wait a while after changing the option above, if there is something odd happening on the host that is holding it open. The file is located in wp-content/plugins/wordfence/tmp/
-Matt R