These simple steps show how you can enable HTTP traffic on iOS
Default only HTTPS (secure) communication is allowed. It is not recommended to Enable HTTP because communication including sensitive data and passwords can be intercepted. You might want to do this if you see an error like: Only https allowed, allow http in OneCalendar security settings
Goto OneCalendar Menu
Select the hamburger menu icon on the top left.
Select the option "Options" in the menu.
Select the option "Security" in the menu.
Set the HTTPS option to OFF.