Zimbra Calendar and Thunderbird Calendar issues

[jwf6911]

Our Zimbra email system (4.5.x) users use Thunderbird and lightning add-on. A user shares his calendar with his secretary with Admin right so that the secretary can add events into his calendar. It had been working fine for a while. However, all of sudden all events in the boss' calendar kind of disappeared from his Thunderbird calendar. And his secretary could not add events any more. But those events did show up when the boss logged into Zimbra web email interface. However when I added a new calendar pointing to a shared Zimbra calendar into the boss' Thunderbird, I was able to see the new calendar events. What might have happened to the Thunderbird and/or Zimbra? I'd appreciate any input!