Addendum: This bug was reported to the Orage developers in 2014 but never fixed.
See https://bugzilla.xfce.org/show_bug.cgi?id=10911 Is Orage abandonware or is this a "won't-fix-bug"? In comment #9 is another workaround https://bugzilla.xfce.org/show_bug.cgi?id=10911#c9 "as I figured out the problem is DTEND tack. When DTEND is before 1970 then this appointment is not displayed. As a workaround I removed all DTEND- lines in ics-file." I think it's much better to patch Orage than fiddling around with the ics-files.