Same problem, but only on the contact forms that had a checkbox. Reverted to a 3 day old backup and everything works fine for now.
I’m having the same issue on all my sites that have updated to WP 4.0 – it was working just fine until the update. I’ve tried updating the plug-in, but it’s already on the most recent version.
Nothing else changed; the update to WP 4.0 is the only variable in play.
