It looks like a timing issue. You are trying to set the visibility before the layer is loaded. Try setting the visiibility property or setting the visibility after the load event.
var ortofotokarte = new Tiled("http://127.0.0.1/MapServer,{visible:false});
map.addLayer(ortofotokarte);
or
var ortofotokarte = new Tiled("http://127.0.0.1/MapServer");
ortofotokarte.on('load',function(){
ortofotokarte.setVisibility(false);
});
map.addLayer(ortofotokarte);