Why does Mail crash on start-up because of some ToDo ?

Question

After upgrading to 10.5.1 (via Software Update), I now have Mail crashing (the correct wording is "unexpectly quitting") at every opening attempt.
I went through many discussions on similar issue, and if I feel better not to be alone with this crashing, none of the proposed solutions has solved my problem. Therefore I elected to post a new topic.
I went into the CrashReporter, and found this information:
+Exception Type: EXC_BREAKPOINT (SIGTRAP)+
+Exception Codes: 0x0000000000000002, 0x0000000000000000+
+Crashed Thread: 13+
+Application Specific Information:+
+-[MetadataManager getAllCalendarStoreData]+
+-[MessageContentController _fetchContentsForMessage:fromStore:withViewingState:]+
+* Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'trying to set a non-ToDo MailboxUid for a ToDo. The MailboxUid for a ToDo must be a ToDosMailboxUid'+ (plus a lot of stuff that I've not copied here.
Actually, before I experienced the first Mail crash, I did create a new ToDo in iCal.
So, I'm now pleased to know that these crashes are due to some ToDo, but I still don't know how to fix that.
Any idea ?

Answer 1

I have now re-instaled Leopard (10.5) from the CD, without updating to 10.5.1
You know what ? Mail crashes again any time I try to read (or preview) a message.
I have just 1 account: my .Mac
I also noticed that the "ToDo" icon in Mail's toolbar is dimmed. Why ? Any reason ?
I'm completely fed-up with this issue. Any help will be very much appreciated. I sent several crash reports to Apple: no answer.
Just for the record, here part of the CrashRecorder log:
Process: Mail [405]
Path: /Applications/Mail.app/Contents/MacOS/Mail
Identifier: com.apple.mail
Version: 3.0 (912.1)
Build Info: Mail-9120100~1
Code Type: X86 (Native)
Parent Process: launchd [324]
Date/Time: 2007-12-27 16:56:57.356 +0100
OS Version: Mac OS X 10.5 (9A581)
Report Version: 6
Exception Type: EXC_BREAKPOINT (SIGTRAP)
Exception Codes: 0x0000000000000002, 0x0000000000000000
Crashed Thread: 18
Application Specific Information:
-[iToolsAccount _synchronizeAccountWithServerWithUserInput:]
-[IMAPGateway _allowClientOperationThrough:]
-[LibraryIMAPStore openSynchronouslyUpdatingMetadata:]
-[MessageContentController _fetchContentsForMessage:fromStore:withViewingState:]
-[MetadataManager getAllCalendarStoreData]
* Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'trying to set a non-ToDo MailboxUid for a ToDo. The MailboxUid for a ToDo must be a ToDosMailboxUid'
Thread 0:
Thread 1:
Thread 2:
Thread 3:
Thread 4:
Thread 5:
Thread 6:
Thread 7:
Thread 8:
Thread 9:
Thread 10:
Thread 11:
Thread 12:
Thread 13:
Thread 14:
Thread 15:
Thread 16:
Thread 17:
Thread 18 Crashed:
0 com.apple.CoreFoundation 0x918d7114 __TERMINATING_DUE_TO_UNCAUGHT_EXCEPTION__ + 4
1 libobjc.A.dylib 0x9681009b objcexceptionthrow + 40
2 com.apple.CoreFoundation 0x918d704b +[NSException raise:format:arguments:] + 155
3 com.apple.Foundation 0x939f00f4 -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 116
4 com.apple.MessageFramework 0x003a5bf4 -[LibraryToDo setMailboxUid:] + 205
5 com.apple.MessageFramework 0x003a5a8d -[LibraryToDo _initWithAccount:] + 169
6 com.apple.MessageFramework 0x003a4f22 -[LibraryToDo initWithCalTask:inAccount:] + 88
7 com.apple.MessageFramework 0x004c97ca -[ToDosCache(Internal) _libraryToDoFromCalTask:addToCache:] + 474
8 com.apple.MessageFramework 0x004c8aff -[ToDosCache libraryToDoFromNewCalTask:] + 49
9 com.apple.MessageFramework 0x002e5b4d -[CalendarStoreInterchange _currentToDosState] + 944
10 com.apple.MessageFramework 0x004c1494 -[CalendarStoreInterchange _retrieveCurrentCalendarStoreState] + 248
11 com.apple.MessageFramework 0x002e16a1 -[MetadataManager(ToDoManagerInternal) getAllCalendarStoreData] + 115
12 com.apple.CoreFoundation 0x918dcb9d _invoking__ + 29
13 com.apple.CoreFoundation 0x918dc588 -[NSInvocation invoke] + 136
14 com.apple.MessageFramework 0x003cf9cf -[InvocationQueue _drainQueue] + 772
15 com.apple.Foundation 0x938de04d -[NSThread main] + 45
16 com.apple.Foundation 0x938ddbf4 _NSThread__main_ + 308
17 libSystem.B.dylib 0x9194f075 pthreadstart + 321
18 libSystem.B.dylib 0x9194ef32 thread_start + 34
Thread 18 crashed with X86 Thread State (32-bit):
eax: 0xa01780f0 ebx: 0x9681007c ecx: 0xa01771a0 edx: 0x006bb000
edi: 0x0ee0b040 esi: 0x005376a0 ebp: 0xb0f0c948 esp: 0xb0f0c948
ss: 0x0000001f efl: 0x00000286 eip: 0x918d7114 cs: 0x00000017
ds: 0x0000001f es: 0x0000001f fs: 0x0000001f gs: 0x00000037
cr2: 0x1be23000