verifikasi gagal
tidak dapat berkomunikasi dengan server
iPhone 6, iOS 12
Are you running macOS server here? If you are, which part of macOS server is involved with the failing connection?
If this is not macOS server but rather involving something else (on iPhone?), is there a particular app involved?
There can be some certificate errors when subscribed calendars are no longer available, as well. Here is how to find and remove those: Delete spam calendars and events on iPhone - Apple Support