I've got a BT Yahoo account so I've got full premium Yahoo services and, apart from the recent mail API issues with Yahoo its been working well. Yesterday though the calendar stopped syncing with an authentication error, but the mail and address book are still working fine.
I tried removing calendar sync to clear the settings, restarted Zimbra desktop and tried to turn calendar sync back on. I now get the message:

In order to use calendar sync, you must upgrade your Yahoo! Calendar Classic to the All-New Yahoo! Calendar.

I have been using the All-New Yahoo calendar for some time now and on the Yahoo web page it shows as the 'Yahoo Calendar New'.

Any one got any ideas what's going on?