Sorry in advance, I am a complete beginner regarding programming, but I cant seem to find an answer to my problem. I made this basic setup from AR.js:
<!doctype HTML>
<html>
<script src="https://aframe.io/releases/1.2.0/aframe.min.js"></script>
<script src="https://rawgit.com/donmccurdy/aframe-extras/master/dist/aframe-extras.loaders.min.js"></script>
<script src="https://cdn.rawgit.com/jeromeetienne/AR.js/1.5.0/aframe/build/aframe-ar.js"> </script>
<a-assets>
<a-asset-item id="rub" src="models/rubcube.gltf"></a-asset-item>
</a-assets>
<body style='margin : 0px; overflow: hidden;'>
<a-scene arjs="debugUIEnabled: false;">
<a-marker-camera preset="hiro">
<a-entity
gltf-model="#rub"
scale=" 0.5 0.5 0.5">
</a-entity>
<a-entity camera></a-entity>
</a-marker-camera>
</a-scene>
</body>
</html>
The problem I get sooner or later with every AR code is that the session starts not with a camera but with a Image. The Image is the png file that is used for the display of the gltf model. When I move the marker in front of the camera the Image disappears and I can see the camera. The marker tracking itself is working.