Sync a caldav calendar and try to delete an event. The whoe caldav account stops syncing and the configuration window shows an error message which says: "There was a problem with the request. The item has not been deleted from the server. The file or folder ... does not exist (0)". The whole account is then broken and won't sync anymore, even if you try to undelete the event. BTW, the same happens exactly with KAddressBook and carddav accounts (I have a caldav and a cardav account in the same server).