Thanks so much. I will look into it. However, I solved my problem like this.
My problem was: I used DELETE request to delete calendar collection to remove all .ics files.
However, when the same user created the account again using same id, the calendar collection was already removed and the user was not able to create events.
Then I found out about : MKCALENDAR / MKCOL requests. So now, I explicitly create calendar collection and handle 403 (forbidden) in my code. This collection is pre-created for brand new users in Darwin Calendar Server.