I am trying to add an external (password protected) WMS to our enterprise portal.
After being prompted to login, I have been getting the error message, that the service does not exist or is not available. I am able to add this service in ArcGIS Pro with the same url, so it does exist and is available.
I have also added all certificates as mentioned here: https://community.esri.com/t5/arcgis-enterprise-portal-questions/problems-adding-external-wms-reference-spain/m-p/1228363#M12857
I have also added the wms domain as trusted server, so I am really out of ideas.
Additional info: I am using the latest ArcGIS Portal 11.2