I found out in the database that the meta_value of the meta_key “wpuf_form_settings” was empty. So I don't know where this happends, but it happends. Maybe this is in combination with the googlemaps object?? I don't know. But I copied the meta_value from a backup i made earlier and pasted it in the empty meta_value VOILA it works again.
Hope you guys can solve this. It happends after saving a form.