iCal - shared calendars not saving settings

I have a user who says he will turn off (uncheck) certain shared cals in iCal and also check ignore alerts on many of them. Over time, he says they will revert back to all being checked and ignore alerts is unchecked. We went in and out of iCal several times today after he made the changes and it didn’t happen. But later in the day, he called and it happened again. Any ideas why this would be happening?

FYI - iCal syncs to our email server (Zimbra) using CalDAV, but these iCal settings should remain on his local machine. There is nothing on our email server that should be causing this.

Thanks for any help.