My Yahoo calendar was working fine in Zimbra (v7.1.2 on Win7x64) this AM as it has been for years, but while away from my desktop it just disappeared w/o any errors at all! The listing for that Yahoo account shows up in the left-hand pane/collapsible calendar tree, but no actual calendars, just the Trash folder. My calendar in Yahoo online was fine, as was my Yahoo "Manage App and Website Connections" permission for Zimbra CalDAV.
Here's what little I could find in the zdesktop.log
2011-11-15 10:34:47,422 DEBUG [btpool0-74] [mid=8;] dav - WebDAV request (depth=zero): /principals/users/userid
2011-11-15 10:34:48,382 DEBUG [btpool0-74] [mid=8;] dav - WebDAV request (depth=one): /dav/userid/Calendar/
2011-11-15 10:34:49,773 DEBUG [btpool0-74] [mid=8;] datasource - Get all mappings for Personal-caldav in folder 10
2011-11-15 10:34:49,793 INFO [btpool0-74] [mid=8;] datasource - Import completed for data source 'Personal'
Multiple changes to my account Setup unchecking and rechecking Synchronize all calendars had no effect. Removing the above Zimbra CalDAV for it to reestablish had no effect, and that permission didn't show up again. Resetting that account in Setup had no effect (though as per another thread it did create a problem w/contacts). My Yahoo calendar is not showing up in Zimbra anymore.
(BTW, I was able to include and remove the calendar for another Yahoo account of mine.)