I have an older MBP and I use my account at this MBP just for administration. I enabled iCloud and synchronized my account. On this MBP is running last Lion version. The mentioned appointment will even not be opened in iCal, in contrast to my actual MBP with OS X 1.8.3, which open the appointment, but taxing me with the mentioned error message.
Anyway having a look at the console messages I found a problem. The problem seems to be the line RDATE in the .ics file.
The exact line is:
RDATE;TZID="W. Europe";VALUE=PERIOD:20130415T110000/20130415T120000
I had a look on other invitations and this line is missing in other invitations. But this line is not the only offending one. I have deleted the line, facing the same problem and I realized the .ics file is really different to other invitations. Finally I have deleted this inviation (anyway it's the past), because this annoying, every few seconds arising error pop-ups, are really dispensable.
Finally what ist worth to be mentioned,
1. so far I'm not connected to the net, I have no problem.
2. No other persons have this problem with the invitation -> this accord me an eccentric status. In this case not desired, because being in a professional environment, my partner doesn't expect I have such problems with a high-tech hardware and software.
3. When I tell iCal to ignore a message I would expect the app respect that. The "ignore" button doesn't effect at all the error-pop-ups.
Thanks a lot for your time you spend for me and my problem!!!
Your hint to use a virgin calendar lead me to the detailed examination of the problem, so far your advice was the right way for me.