This form is on an index page, and you are using current page submit as the trigger. Beware of current page submit triggers on index pages.
I bet what is going on is this:
In testing, you are not adding the page name to the URL:
when the page name is left off, the current page submit trigger wont work.
even though the server knows to load the index page when the page name is left off, the php variables used by the currant page submit trigger wont match. change the trigger too use the button instead.