Why does CiviCRM render a validation failure without the Wordpress theme?
I can't add a comment (doh, stupid reputation requirement) so I'm answering when Thomas Jaunism wrote: "Why would CiviCRM be trying to load the page.php and not the front-page.php that the form is on?"

The Civi plugin loads the WordPress theme's page.php template because many (if not most) themes define a homepage template which does not contain the_content() but rather collections of widgets and archive views. Loading page.php has proved to be the most reliable way to show Civi content on the front end.

If you want Civi to load a different template, there is a filter for this called civicrm_basepage_template. You can therefore load a different template with code such as the following in your functions.php file:

add_filter( 'civicrm_basepage_template', 'use_my_own_template_for_civi' );
function use_my_own_template_for_civi( $template ) {
    if ( some condition is met ) {
        return 'your-template-name.php';
    }
    return $template;
}