Hi Michel:
Thanks, for the response.
Correct on the PRAM, but it was a suggestion in a thread that had some merit. The issue in my case occurred after a power outage. The MacMini Lion is the main iCal which syncs to iCloud and backs up via TimeMachine. I had been updating the events from my iPhone on the light rail on my way home, maybe 5 of them (since iCloud / iPhone / iCal updates are so slow), and a few of them were repeating events, and these were the only ones causing the 500 server error on iCloud.
What likely happened is these events were in the midst of an update when the power went out, and just got out of sync with the iCloud.
I contacted Apple Support and got through to an engineer, and after logging into a different profile on the MacMini, subscribing to iCloud, and syncing, the issue with the select events throwing 500 server errors when modifying them persisted. The Apple agent recommended I Archive iCal in the main profile and delete all events and re-import from the Archive. Did so, and it got a bit more bizarre.
After deleting the calendars from iCloud and re-importing, I would see the calendars and events in iCal on the MacMini for a few seconds (2-3), then they'd disappear. :-) I did this several times until I realized the iCloud had authority and any re-imports of the archive were going to continue to have the calendars / events deleted immediately after iCloud saw them and sent back to the client these calendars were deleted.
So, I did the following:
I launced Terminal, I did the following to tar up the iCal Library created during the subscription to iCloud via the other MacMini profile:
sudo su -
cd /Users/other-profile-user/Library/
tar czvf /Users/main-profile-user/Desktop/other-profile-user.ical.20120510.tgz Calendars/
exit
cd /Users/main-profile-user/Library/Calendars/
tar xzvf ~/Desktop/bond.ical.20120510.tgz && find ./Calendars -type f -exec chmod 400 {} \;
logout
What that terminal business did was created a complete archive of the unsync-ed Calendars directory in the main user profile and the double ampersand ( && ) in Unix tells the bash shell (which is the api in terminal, unless the shell is changed) to process the first command and immediately execute the next command without delay. In this case the find command instructs the shell to find all files in the Calendars folder and make them read-only (chmod 400). This prevented iCloud from writing it's empty calendar version of the iCal store to the local cached version, thus retaining the calendars and events in iCal.
After restoring my calendars in a read-only state, I went back into iCal and exported each calendar individually which only exports the events, not the iCloud profile and credentials, which Archive does, so I could remove the Calendars in iCal (after chmod -R 644 * /Users/main-profile-user/Library/Calendars/ to make the calendar items / events writeable again in terminal), then re-subscribe to iCloud, and import these exported calendars individually back into iCal, and sync with the iCloud.
Very complicated, to be sure, but I'd have lost 8 years of calendars without following this procedure.
If I had just exported the individual calendars, rather than archiving them before deleting them from the iCloud, this procedure would have not required Terminal.
Best Regards,
Van