Certificate and server issues
Why do I keep getting things like this certificate is not supported or I get it this is not available on the server or server cannot be found. How do I fix this
Do you have an unrecognized or unneeded subscribed calendar configured?
If so, remove it. Here is how: Delete spam calendars and events on iPhone - Apple Support
This was posted in macOS Server. Are you using macOS Server here? (I am going to assume probably not, and that you’re using an iPhone, iPad, or a Mac, and not macOS Server. But if you are using macOS Server, those configurations do require server certificates and certificate updates. See the certificate-related messages in Server app as a starting point.)