hey, we noticed the issue when users linked to calendars with sub-calendars/.ics . In your client's case have them try just deselecting the calendar with sub-calendar/.ics ( which may also be corrupt ). If it works, then they should choose just the main-level calendar and not the sub-calendar.
It appears IE might have an issue with the javascript used in parsing sub-level calendars or .ics files...dunno, zimbra team?
