Is Your app in Your personal workspace or in a Qlik Sense For Business workspace? Is the app published to a stream? Is problem seen in all apps? Can You create a new story in the app where You see the problem and just add an object such as a triangle or something and export that story?
Can You duplicate the story and try to publish the duplicate? If that also fails remove content from the copy, 1 thing at a time until You find the one causing the problem? Then duplicate the original once more and remove only the one causing the problem and then recreate that object by taking a new snapshot of it and add to the copy and try to export again?