Calendar sync. error during account set up with Yahoo! mail accounts

[isau22]

Newbie here drawn to Zimbra desktop by capability of full-offline working with Yahoo! mail accounts (and why isn't this IMAP??).

Have set up two Yahoo! email accounts but when try to select calendar sync. and activate account I get the following error message:

Unable to access calendar service. You will not be able to sync your calendar.
Deselecting the calendar sync option allows the account set-up to go ahead.

But I want my calendars! If this isn't going to happend then I'm afraid Zimbra gets a 'pants' rating, Yahoo! will be ditched and it looks like I'll have to go elsewhere.

I see that others have had similar problems with other non-Yahoo! email accounts but nowhere a solution - is this a known problem ...or a 'feature'?

Anyone know how to fix it?