calendar

I get this message in Calendar apps.... "The URL https://caldav.calendar.yahoo.com/principals/users/pkroner@pacbell.net///caldav.calendar.yahoo.com/principals/users/pkroner@pacbell.net/ encountered HTTP error 404. Make sure the URL is correct."

This is new after years of streaming, etc wit all appps up to date. OSX 10.8.3 and using an iPod, Ipad, Iphone.....

I can't tell whether this is an Apple system problem or some glitch ion my system.....