The wp_safe_redirect() function doesn’t work for external URLs unless the domain name is included in the allowed redirect hosts. Can you please add a filter to automatically add all the domain names we’ve set to redirect to this list of allowed hosts using the 'allowed_redirect_hosts' filter? See here for an example.
On line 28 of hidden.php, change this:
add_action('plugins_loaded', 'contact_form_7_hidden_fields', 10);
to this:
add_action('plugins_loaded', 'contact_form_7_hidden_fields', 11);
