These simple steps show how you how to disable certificate validation on Windows
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.
Goto OneCalendar Menu
Select the "settings" menu icon on the bottom right.
Select Options
Select the option "Options" in the menu.
Select Security
Select the option "Security" in the menu.
Disable Certificate validation
Set the option "Validate certificates" to OFF.