I upgraded the software on my iPhone and my Yahoo Calendar stopped syncing via CalDav
Here's what I did to fix the problem.  I got the answer from this forum post.


Delete CalDav Yahoo calendar sync profile in the iPhone settings.
Go to Safari settings and click Clear History and Clear Cookies and Data.
Turn iPhone off.
Turn iPhone on.
Re-add you Yahoo Calendar CalDav account.


To re-add your Yahoo Calendar CalDav account:


Go to Settings
Click Mail Contacts, Calendars
Click Add Account Option
Click Other
Click Add CalDAV Account
Server: https://caldav.calendar.yahoo.com
Enter you Yahoo user name and password.
Click Save.