Here is their full explanation which I hope will show that there is hopefully a simple getaround, the only problem being that I lack the knowledge to apply the fix...
You are encountering this issue because you had installed a cache plugin called wp super cache and it is caching the contact form.
You need to set wp super cache not to cache the contact form, you can do this by entering the contact form url in wp super cache setting page.. somewhere that says do not cache url containing…
In the webpage html source code of the contact form, there is a hidden field called _wpnonce for security check.
<input type="hidden" id="_wpnonce" name="_wpnonce" value="5acb700fe6" /><input type="hidden" name="_wp_http_referer" value="/contact" />
When you post the form, the contact form code will check and verify this _wpnonce value and the http referer, so as to make sure that it is data submit from your webpage and not any other mail spamming program.
If the _wpnonce value is different, it will show you the error message “Are you sure you want to do this?” and terminates the program immediately after.
This is what you actually encountered…
When you submit the form and coincidentally wp super cache regenerates a new cache file, the _wpnonce value posted will be different from the verification value.
This will trigger the WordPress error message.. which is .. “Are you sure you want to do this?”
wp super cache only regenerates the cache file after certain period of time, this explains why you encounter this issue randomly.
So, can I set up WP Super cache not to cache the contact form url, and how do I do that?
[No bumping. If it's that urgent, consider hiring someone.]