I've had this happen a few times. When shortcodes fail, it is either because the plugin that drives them has become deactivated/failed or because the text representing the shortcode was turned into HTML.
Go to the Post/Page editor and look at the Kontakt page, click on the HTML tab to edit in HTML view and look at the page content. Do you see it marked up with HTML tags around it? Or is it put in a line with other text?
You say it was working until recently, has anything been introduced in that time? A plugin that does code validation for example?