Hi Zoltán,
the problem with those zoom levels is: you can only specify a max. number
of zoom levels for the OpenLayers API per map.
Unfortunately, the maximum zoom layer for the hill shading layer is smaller
than the one for OSM and Google maps.
I could build an option so you can specify the number of zoom level in the
plugin configuration, but the users of your site can turn the hill shading
layer off interactively, and in that case you would set a limit that could
"do better" with hill shading switched off...
I'm not sure how to solve this *completely*, but that new parameter could
at least avoid the pink map problem - any better ideas??
cheers, Frank