iCal subscriptions disappearing

I maintain Macs for about a dozen users who share their calendars via iCal (Lion 10.7.5). For some reason the calendar (iCloud) subscriptions randomly become invalid and the little triangle warning is displayed. This happens with no apparent pattern on most of the Powerbooks. When I delete a subscription and resubscribe, the calendar works fine again...until the subscription stops working again, a month or week later. It's annoying to have to resubscribe over and over.

Has anyone else seen this and know of a fix? A Google search turned up nothing...