One of my users, on a MacBook Air running OS X 10.6.8, has her Zimbra e-mail account synced to iCal (via the Zimbra connector). Recently, one of her calendars disappeared from the Zimbra web GUI, but is still visible in iCal.

I tried having her log out and log back in to the web GUI - no luck.
I tried re-indexing her account - no luck.

We're running Zimbra Collaboration Suite v6.0.13 on the backend.

Any thoughts?