Thank you for your fast support. Hhhmmm – I am not a PHP coder, but I've integrated your shortcode in header.php as you cann see on attached image.
1.) I've found wp_head() in header.php line 119.
2.) Plugin is activated
3.) I've added shortcode as html in page (not in template), so you can see, that both variations are not working: http://inappixcom.staging.wpengine.com/domains/ – first code on top (black navigation) is code in template (do code). Second added with wysiwyg.
If I copy/paste default WP search form, search form is working fine. Strange.
Thank you, Marcel