Calendar can't verify the identity of this server 

'p12-caldav.icloud.com'

as you can see in the photo, if you expand the tab called 'trust' you see who generated this certificate to access your calendar. Hackers from China. select 'Never trust' and then continue. This blocks the certificate forever. Make sure then to scan your mac.

ArcadyanGZ

www.httpd.org

arcadyan@arcadyan.com ArcadyanGZ

all are odd labels.