Thank you , ligfietser, especially for your patience.
"Hmm is this the same problem as you already asked in another topic: " In principle yes. But I misunderstood your last entrance under this topic (“The bug of Basecamps Base map superposes the OSM map has just been traced and solved by developer Steve from mkgmap, so sooner or later the results will be visible in all the OSM maps that are produced with mkgmap.”) and thought, in the newest version of mkgmap this problem would be solved now. And the downloads from raumbezug are produced under a former version of mkgmap. So I tried to produce a map myself with the result as reported here.
But you reminded me now to your advice with the empty basemap. I forgot this and lost the old settings because I switched from basecamp to basecamp 33 beta which is installed in other folders.
As I said already in the other entrance, I am a beginner in this area. I decided now to be a little more patient and to by now first a book to learn how Java works (I made my first programming exercises on a ZUSE Z22 and Java was only an island at that time).