I get this constantly. If I click OK, then right click the Attention icon by the calendar name, select "Show CalDAV Queue" and click the Go button, it syncs fine... it's quite annoying seeing the iCal icon bounce incessantly in the dock. Why can't it sync on it's own? None of my other calendars seem to have this problem and all the settings are as Calaboration has set them ?
I didn't find the right solution from the Internet.