server responded "403" to operation CalDAVWriteEntityQueueableOperation

Attempting to add an appointment to iCal from an .ics file that is downloaded from an email, I am told that "Access to -- and the name of the meeting -- in "Work" (the account of iCal I wanted it in) in account (my Apple id) is not permitted. The server responsed '403' to operation CalDAVWriteEntityQueueableOperation."

I have the option of going offline or reverting to server. If I go offline the appointment stays -- until I go online again. If I revert to server, it disappears.

This has been going on for about a year now so I have been adding meetings by hand.

However: This morning I was greeted with an appointment on my iCal that had arrived via a SPAM message in my Inbox that I had not even read, let alone clicked on! Hoping this meant that the .ics curse had been magically lifted, I tried to add an appointment today -- with the above result.

OS X 10.7.5

iCal 5.0.3

Mail 5.3