See for example: https://osmetadata.astuntechnology.com/geonetwork/api/collections?f=xml the protocol should be HTTPS, not HTTP; if you copy any link (e.g. http://osmetadata.astuntechnology.com:443/geonetwork/api/collections/main?f=xml) and change the protocol manually to https then you can access it just fine, otherwise, you get an error.
I checked the GeoNetwork settings and the protocol of the catalogue is set to https, so I am at a loss as to where this protocol is coming from.
I am running the service in docker, so is this something I need to set in the .yml definition?
The service definition can be seen here: https://github.com/AstunTechnology/os-custom-geonetwork/blob/os-4.2.x/docker-compose.yml#L100 - The GN_PROTOCOL
variable is also set to https
in the env file on the server.