I do not understand why the markers disappear, zoom 6, only in this area map.
Do you have an idea?
This is a known issue of OpenLayers, there is a known workaround is to add:
to your vector layer configuration.
I have tested it with firebug in your page. With that line added, it works all right.
The reason is that the default SVG renderer has a limited coordinates range.
For more details please checkout the issue page: https://github.com/openlayers/openlayers/issues/668
Hmm, It looks fine in my browser (IE9). Have you tried using a different browser? Have you tried clearing your browser cache?