Sync Services malfunction: attempt to insert nil

Question

I've been using Entourage (Office 2008) and iCal for years, with calendar entries in Entourage syncing to iCal (and thence to my iPhone and back again).
Recently, the syncing process has gone haywire.  By and large, Sync Services is only updating old calendar entries (2008 and earlier) into iCal.  Some (but not all) repeating events get synced, so I see a handful of birthdays in 2011 and beyond in Cal. But nothing else more recent than 2008. It appears that the syncing process starts, but then aborts for reasons unknown.
The console log is chock full of Sync Services errors. The following is but a snippet.
9/30/11 9:56:11 AM          iCalExternalSync[4950]          [ICalExternalSync ]Encountered *** -[NSCFSet addObject:]: attempt to insert nil
9/30/11 9:56:11 AM          com.apple.syncservices.SyncServer[4122]          2011-09-30 09:56:11.066 iCalExternalSync[4950:903] [ICalExternalSync ]Encountered *** -[NSCFSet addObject:]: attempt to insert nil
9/30/11 9:56:11 AM          iCalExternalSync[4950]          [ICalExternalSync ]#2  0x00007fff88e3e5d7 in +[NSException raise:format:arguments:] ()
#3  0x00007fff88e3e564 in +[NSException raise:format:] ()
#4  0x00007fff894ffe53 in -[NSCFSet addObject:] ()
#5  0x0000000100015eec in ?? ()
#6  0x000000010000fc7d in ?? ()
#7  0x00007fff83980457 in -[ISyncConcreteSessionDriver _pullPhaseOfSession] ()
#8  0x00007fff8397cf91 in -[ISyncConcreteSessionDriver _sync:] ()
#9  0x000000010000f0ec in ?? ()
#10 0x000000010001e9d0 in ?? ()
#11 0x0000000100001e50 in ?? ()
9/30/11 9:56:11 AM          com.apple.syncservices.SyncServer[4122]          2011-09-30 09:56:11.257 iCalExternalSync[4950:903] [ICalExternalSync ]#2  0x00007fff88e3e5d7 in +[NSException raise:format:arguments:] ()
9/30/11 9:56:11 AM          com.apple.syncservices.SyncServer[4122]          #3  0x00007fff88e3e564 in +[NSException raise:format:] ()
9/30/11 9:56:11 AM          com.apple.syncservices.SyncServer[4122]          #4  0x00007fff894ffe53 in -[NSCFSet addObject:] ()
9/30/11 9:56:11 AM          com.apple.syncservices.SyncServer[4122]          #5  0x0000000100015eec in ?? ()
9/30/11 9:56:11 AM          com.apple.syncservices.SyncServer[4122]          #6  0x000000010000fc7d in ?? ()
9/30/11 9:56:11 AM          com.apple.syncservices.SyncServer[4122]          #7  0x00007fff83980457 in -[ISyncConcreteSessionDriver _pullPhaseOfSession] ()
9/30/11 9:56:11 AM          com.apple.syncservices.SyncServer[4122]          #8  0x00007fff8397cf91 in -[ISyncConcreteSessionDriver _sync:] ()
9/30/11 9:56:11 AM          com.apple.syncservices.SyncServer[4122]          #9  0x000000010000f0ec in ?? ()
9/30/11 9:56:11 AM          com.apple.syncservices.SyncServer[4122]          #10 0x000000010001e9d0 in ?? ()
9/30/11 9:56:11 AM          com.apple.syncservices.SyncServer[4122]          #11 0x0000000100001e50 in ?? ()
9/30/11 9:56:11 AM          iCalExternalSync[4950]          [ICalExternalSync ]NSException name:NSInvalidArgumentException reason:*** -[NSCFSet addObject:]: attempt to insert nil
9/30/11 9:56:11 AM          com.apple.syncservices.SyncServer[4122]          2011-09-30 09:56:11.258 iCalExternalSync[4950:903] [ICalExternalSync ]NSException name:NSInvalidArgumentException reason:*** -[NSCFSet addObject:]: attempt to insert nil
9/30/11 9:56:32 AM          iCalExternalSync[5076]          [ICalExternalSync ]Another sync will be executed to insure the com.apple.ical.calendars.TaskOrder singleton is correct
9/30/11 9:56:32 AM          com.apple.syncservices.SyncServer[4122]          2011-09-30 09:56:32.531 iCalExternalSync[5076:903] [ICalExternalSync ]Another sync will be executed to insure the com.apple.ical.calendars.TaskOrder singleton is correct
9/30/11 9:56:35 AM          iCalExternalSync[5076]          [ICalExternalSync ]Encountered *** -[NSCFSet addObject:]: attempt to insert nil
9/30/11 9:56:35 AM          com.apple.syncservices.SyncServer[4122]          2011-09-30 09:56:35.319 iCalExternalSync[5076:903] [ICalExternalSync ]Encountered *** -[NSCFSet addObject:]: attempt to insert nil
The repeated messages about "attempt to insert nil" seems suspect, but I'm not sure what to do about it.
Syncservices is generating many megabytes of log files, starting a few days ago.
My troubleshooting so far:
reboot OSX
disable the Sync preference in Entourage ('Synchronize events and tasks'), and re-enable
quit/ relaunch Entourage (many times)
quit/relaunch iCal (many times)
rebuild the Entourage database (in case it had become corrupted)
delete the iCal prefs and database, per an article at http://www.office.mvps.org/syncservices/ical.html which said:Deleting these files should restore your default iCal calendars:
~/library/preferences/com.apple.iCal.AlarmScheduler.plist
~/library/preferences/com.apple.iCal.helper.plist
~/library/preferences/com.apple.iCal.plist
and delete the contents of
~/Library/Calendars/Calendar Cache
re-enable the Sync preference in Entourage. After doing so, The Entourage calendar in iCal reappeared, but once again, only really old calendar entries from Entourage were transferred over. Sync Services announced there was a large number of conflicts that needed to be resolved.  I granted permission, but the listed events didn't appear.
I don't have MobileMe. I don't sync to Google Calendars or anything else.
My iPhone is now stranded - I don't dare sync it with iTunes as the calendar information which is in it will get clobbered.
Any ideas on how to restore proper syncing?

Answer 1

OP reporting more information.
I used Time Machine to go back to last week when syncing was working, and restored all the sync-related files that I could find:
~/Library/Application Support/Calendars
~/Library/Application Support/iSync
~/Library/Application Support/SyncServices
~/Library/Application Support/iCal
~/Library/Application Support/PreferencesSync
~/Library/Preferences/iCal.plist
~/Library/Preferences/iSync.plist
This had the beneficial effect of restoring most of my event data from iCal into Entourage. (it's pretty much the view in iCal that I had last week). However, syncing is still not working. Any changes made to events on either side (Entourage or iCal) don't get synced across.
Every time SyncServices runs, it generates thousands of errors in the log. It seems to be logging in Debug mode, as if it knows there's something wrong. I've scanned through the logged messages, but nothing jumps out as being the cause of the non-syncing.
Perhaps I'll have to reinstall OSX from scratch :-(