I copied the original project again and before I opened the schematics I went to the layout first. The problem was not there now. Then I saved the converted layout, went to the schematics, saved again, and again back to the layout. Still the problem was not there.
But another one problem appeared. All self-defined footprints for mounting holes were completely filled with the “silk layer”. So opened the footprint, switched from “Durchgehende Bohrung” to something else and back to “Durchgehende Bohrung” and the footprint was displayed fine again.