I would like to to use the WMS url from https://www.alegrodialog.de/profiles/markaspot/modules/mark_a_spot/modules/markaspot_wms/markaspot_wms.js which is about the ALEGrO DC power line.

var alegro = L.tileLayer.wms("https://map-proxy.zebralog.de/maps.zebralog.de/cgi-bin/maps-amprion", {
  layers: 'zebralog',
  service: 'wms',
  version: '1.1.1',
  format: 'image/png',
  srs:"EPSG:3857",
  attribution: "Alegro Trasse",
  transparent: true
});

Shouldn't it simply be https://map-proxy.zebralog.de/maps.zebralog.de/cgi-bin/maps-amprion ? Or more elaborate:

wms:https://map-proxy.zebralog.de/maps.zebralog.de/cgi-bin/maps-amprion?FORMAT=null&VERSION=1.1.1&SERVICE=WMS&REQUEST=GetMap&LAYERS=zebralog&STYLES=&SRS={proj}&WIDTH={width}&HEIGHT={height}&BBOX={bbox}

But I've tried with QGIS and JOSM and the URL is dead.

Have I done a mistake or is the WMS endpoint really blocking me?