Access to iCal Not Permitted

I am running Mac OS X Server 10.7 with calendar server on it. When some of my clients received an invitation on a mail attachment which is a .ics file, and then include this event to the iCal, the error below will appear and I don't know why it incurs. So can someone please help me in resolving it?

Request Error
Access to calendar is not permitted.
The server responded
"HTTP/1.1 403 Forbidden"
To operation
CalDAVAddCalendarQueueableOperation
