I would still like to know how you created this appointment, but I do know why Outlook 2003 complains, and it looks like it is because of a bug in the way Outlook 2003 processes these .ics files.
The "period" in this .ics is 1, which makes sense -- it occurs every day. The interval is 1 in the .ics file. Whereas Outlook 2007 treats this as a weekly occurrence, occurring every week on Monday thru Friday, Outlook 2003 treats this as a daily occurrence, occurring every weekday. But for daily occurrences, the period is supposed to be stored in minutes (i.e. 1440), but in this case, the period is stored in days (1), so our code does its calculations, and determines that this is a bad appointment.
Not sure I can code around this. Depends on what I find out about how you created this appointment.