My experience using Ical from my IMac is that the Caldav server response is at best unreliable and totally unresponsive at other times. The service normally fails for a day or so but currently has not been available since the start of this week with no archive downloads to my calendar.
One of the error messages I get is :-
The URL https://caldav.calendar.yahoo.com/principals/users/**********/ encountered HTTP error 404. Make sure the URL is correct.
Is this a known problem and is there a solution.
I have (had) exactly the same problem and error message since early May. On Friday I rang Apple Support and they talked me through a way of removing the problem by changing to using my iCloud account for my Calendar. Very simple just export all the calendar entries from your Web version of your @btopenworld.com calendar and import them into your iCloud account on your desktop. Then just disable the calendar synchronising setting for your @btopenworld.com account in preferences. No synchronising, no error messages!
Might not work for everyone, but works for me. I have also moved all my Notes from @btopenworld account to my iCloud account and disabled that attribute aswell as I was suffering from patchy syncronising for these.
Working on moving all my real world contacts to the iCloud account now as I have an @btopenworld.com sub account that has only every syncronised once with my iMac, and constantly gives error messages.
Anyway the nub from Apple support was that I was the 4th person on Friday to call with exactly this CalDav problem, it is not anything that Apple can do to fix, only help bypass, as it is a Yahoo problem.
Hope that gives you some options.
(I am now working on how to move my e-mail from Btopenworld.com, but that has wider impact so will take longer)
Many thanks for your response, useful to know it is a known issue.
One thing I am not sure about is how I retrieve my calendar entries from the Caldav server when I cannot access it, my desktop calendar has no appointments etc in it, it is completely blank.
Thanks again for your help, am I missing something in the process?