Ok. I have enabled garmin maps in maps.xml; OSM maps are enabled as well. In navit - settings I can choose between the two maps. The path to the garmin map set is correct.
The map set consists of lots of small maps (*.img), a basemap.img, and a file product.tdb. The path is directed towards the basemap.img file.
(/home/user/navit_maps/basemap.img)

Osm maps are working perfectly.
In navit I disabled OSM maps, and enabled the garmin maps, but no map becomes visible.

I have tested the same mapset in QLandkarte GT, where it works perfectly.

What am I doing wrong ?