delete all active and cached form sessions
Using Jamroom
when you open a form, a form_key is set in the form. Its unique each time the form is displayed. This key is there when the form is displayed on your site, and is expected to be there when the filled in form is submitted. If its not you get an "invalid form_key error, please refresh and try again" type error.
If you reset the cache while someone is filling in a form, their key becomes invalid and the form submission will not be accepted.
The purpose of the key is to prevent form submissions coming from nasty people trying to trick the system into doing things its not supposed to do.