These simple steps show how you how to disable certificate validation on macOS
Certificate validation is turned on by default. It's not recommended to turn it off, it significantly increases the risk of Man in The Middle (MiTM) attacks. You can turn it off for troubleshooting purposes.
Select Options
Select the OneCalendar menu on the top left, and select the option "Options".
Select Security
Select the option "Security" in the menu.
Disable Certificate validation
Set the option "Validate certificates" to OFF.
NOTE: This does not work for all calendar types.