ICal problems with BT/Yahoo server access on my IMac

A couple of weeks ago ICal on my IMac started not to access the ICal server via BT/Yahoo, wrong URL was the error message. I have now moved ICal on my Imac and IPad to ICloud and no longer get error messages.

However although the IPad works correctly the calender on my IMac is not being updated with historical appointments, it only shows new information. If I renter older saved appointments on my IMac they also appear on my IPad effectively as duplicates.

Any suggestions please?