Usually when an override does not work there is a secondary issue. Who issued the certificate for the domain when you click view? Your antivirus perhaps? The fact it is for a different domain is itself something of an issue as I would assume Apple know how to configure a web domain. But perhaps not. However my test indicates a valid certificate without any issue as to trust is being sent to general web ports. https://www.immuniweb.com/ssl/p53-caldav.icloud.com/nz3MN4VL/ But it is a server cluster, so could conceivably contain a bed server, and you keep getting it.