Cannot add event to iCal, server says "forbidden"

CalDAV error is not allowing me to add an event to my iCloud calendar from EmClient.  Server Access Control would suggest that I have “write” permissions, but operation fails.
Screenshot here:

I must admit I’m a little frustrated by all the time I’ve wasted looking for information in these forums.  My search queries all result in thousands of hits, but every thread ends in a request for private emails, effectively taking the discussion offline. No followup or resolution for the public.  For every hundred dead-end threads here, we would benefit more from a single open conversation that takes us to an actual conclusion.  
In the interim, please let me know how I can resolve this problem.  

Hi Theodore, is this by any chance a shared calendar folder, created on a different iCloud account?
What version of eM Client are you using?

Thank you,