If I try to use this WMS:
https://www.ogd.stadt-zuerich.ch/wms/geoportal/Historischer_Uebersichtsplan_der_Stadt_Zuerich_um_1900
(which works in QGIS), I get just the generic error msg "URL is not a valid WMS server (error loading WMS Capabilities)".
Sometimes it's a leading blank in the URL which gives the same error.