I found a solution by further testing:
wm = WebMap()
wm.basemap.clear()
wm.basemap.baseMapLayers[0].clear()
webAtlasDe = '{"id": "WebAtlasDE_1637","layerType": "ArcGISTiledImageServiceLayer","url": "https://myserver.de/arcgis/rest/services/WebAtlasDE/ImageServer","visibility": true,"opacity": 1,"title": "WebAtlasDE"}'
wm.basemap.baseMapLayers[0].update(json.loads(webAtlasDe))
wm.basemap.update({'title':'WebAtlasDE'})
webmap_item_properties = {'title':'WebAtlasDE', 'snippet':license_text, 'tags':['basemaps'}
service = wm.save(webmap_item_properties)