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So in the web client I go to "Calendar", "New Calendar", choose "Sync appointments from remote calendar" and enter the URL there. I then always get error messages like "The Following URL is currently not readable: http: ..." or "Appointments could not be retrived. MAke sure the following URL is a valid ICAL feed: http:// ..."
The latter feed gets a "403 Forbidden" when Zimbra tries to fetch it.
The former one (the German holidays) works fine for me on my 4.5.2 system, which leads me to believe that it's a problem with bug 13117, fixed in the upcoming 4.5.2 release.
The grouphup.com url was mudged because the information on that calendar is considered private. Interestingly I can NOW import a calendar from that URL. So I assume grouphup.com (Which is http://www.backpackit.com/ ) had an issue regarding ics generation.