Broken Form -- Not related to Theme/Plugin Conflict.

I have run into this exact issue. Viewing the preview of any form in our site displays a 404.

This is happening because of the way gravity forms bootstraps wordpress in preview.php.

require_once (preg_replace("/wp-content.*/","wp-blog-header.php",__FILE__));

Loading wp-blog-header.php causes wordpress to instantiate the WP object, which on initialization parses the request and returns a 404 HTTP Status since the preview.php uri does not match any permalink, page or post known to wordpress. You can observe this by attaching a filter to 'status_header' and printing out what wordpress returns:

add_filter('status_header', 'print_header');
function print_header($status_header) {
    echo $status_header;
    return $status_header;
}

Prints: 'HTTP/1.1 404 Not Found' for any gravity form preview.

We are running nginx as our web server, and it is configured to handle 404 errors. So, as soon as it gets a 404 response from php/wordpress, it ignores anything else and returns its built in 404 page. The original submitter may have Apache configured similarly.

A simple solution for this is to bootstrap wordpress differently in preview.php.

require_once (preg_replace("/wp-content.*/","wp-load.php",__FILE__));

This bypasses initiating the WP object and fully loads wordpress. WE have implemented this solution and haven't experienced any issues.

A second solution would be to set the response header in preview.php to '200' before sending any content, thus overriding the header that wordpress had set.

status_header('200'); //wordpress specific

Hope this helps others.

Brian