Hi There,
I did some looking around and found the theme is the problem.
I did a simple post to the themes demo page on http://demo.touchsize.com/codmark/wp-admin/admin-ajax.php
with the parameter "action" and got the same message.
For now, disable ajax on the form: Edit the form, go to General Settings, and uncheck Ajax Submissions and save. This will put the form into page reload on submit.
Then you can go post a support request at the theme developers support site here: http://support.touchsize.com/index.php?module=tickets&sec=submit.step1