I had this problem on OSX server 10.6.8 and a couple of users sharing their calendars thru iCal.
After a Time machine backup restore, the owner names were altered on the /Library/CalendarServer/ and thus the user were not able to write their local machine cache content onto the server.
The problem was probably triggered by alteration of folder (& content) permissions, so the Admin can see the file contents and identify the user's calendar data for restore.
It appears that the folder owner has to be specifically "_calendar"; allowing Owner Group Others full Read Write rights was NOT able to circumvent this permissions issue.
I also reset the user's passwords, turned on / off the ical server, emptied the ~/Library/Calendars/Calendar Cache, as mentioned elsewhere but that did not help.
Your mileage may vary, and as always, make backups and take precautions before proceeding.