It crashed again. This time I was running with he "--debug" flag. Here is the error output:

.
.
.
[DEBUG 15:21:05.254] Saving 'Ubuntu Oneiric Ocelot: Post-install'...
[DEBUG 15:21:05.390] Note saved or deleted within a minute of next autosync...resetting sync timer
[DEBUG 15:21:05.857] Saving 'To Do: Some Day'...
[DEBUG 15:21:05.896] Note saved or deleted within a minute of next autosync...resetting sync timer
[DEBUG 15:22:05.898] BackgroundSyncChecker: Checking server for updates
[DEBUG 15:22:05.899] Building web request for URL: https://one.ubuntu.com/notes/api/1.0/
[DEBUG 15:22:08.903] Building web request for URL: https://one.ubuntu.com/notes/api/1.0/user/
[DEBUG 15:22:10.084] BackgroundSyncChecker: Detected that sync would be a good idea now
[DEBUG 15:22:10.084] SyncThread using SyncServiceAddin: Tomboy Web
[DEBUG 15:22:10.085] SilentUI: SyncStateChanged: Connecting
[DEBUG 15:22:10.130] SilentUI: SyncStateChanged: AcquiringLock
[DEBUG 15:22:10.131] Building web request for URL: https://one.ubuntu.com/notes/api/1.0/
[DEBUG 15:22:10.444] Building web request for URL: https://one.ubuntu.com/notes/api/1.0/user/
[DEBUG 15:22:11.370] 8
[DEBUG 15:22:11.370] SilentUI: SyncStateChanged: PrepareDownload
[DEBUG 15:22:11.370] Sync: GetNoteUpdatesSince rev 758
[DEBUG 15:22:11.370] Building web request for URL: https://one.ubuntu.com/notes/api/1.0/op/?include_notes=true&since=758
[DEBUG 15:22:13.144] Sync: 0 updates since rev 758
[DEBUG 15:22:13.144] Building web request for URL: https://one.ubuntu.com/notes/api/1.0/op/
[DEBUG 15:22:15.511] SilentUI: SyncStateChanged: PrepareUpload
[DEBUG 15:22:15.511] Saving 'Ubuntu Oneiric Ocelot: Post-install'...
[DEBUG 15:22:15.516] Saving 'Ubuntu Oneiric Ocelot: Post-install'...

(Tomboy:22598): Gtk-CRITICAL **: IA__gtk_tree_model_row_inserted: assertion `path != NULL' failed
Stacktrace:

  at (wrapper managed-to-native) Gtk.ListStore.gtk_list_store_set_value (intptr,intptr,int,intptr) <IL 0x00012, 0xffffffff>
  at Gtk.ListStore.SetValue (Gtk.TreeIter,int,GLib.Value) <IL 0x00021, 0x0009f>
  at Gtk.ListStore.AppendValues (System.Array) <IL 0x0002f, 0x0010b>
  at Gtk.ListStore.AppendValues (object[]) <IL 0x00002, 0x00027>
  at Tomboy.NoteRecentChanges.UpdateResults () <IL 0x000f8, 0x003bf>
  at Tomboy.NoteRecentChanges.OnNoteSaved (Tomboy.Note) <IL 0x00013, 0x0003f>
  at (wrapper delegate-invoke) <Module>.invoke_void__this___Note (Tomboy.Note) <IL 0x00056, 0xffffffff>
  at Tomboy.NoteManager.OnNoteSave (Tomboy.Note) <IL 0x00012, 0x0002c>
  at Tomboy.Note.Save () <IL 0x000f0, 0x00244>
  at Tomboy.Note.SaveTimeout (object,System.EventArgs) <IL 0x00001, 0x0001f>
  at Tomboy.InterruptableTimeout.TimeoutExpired () <IL 0x00018, 0x00029>
  at GLib.Timeout/TimeoutProxy.Handler () <IL 0x0000d, 0x0003a>
  at (wrapper native-to-managed) GLib.Timeout/TimeoutProxy.Handler () <IL 0x00029, 0xffffffff>
[ERROR 15:22:15.582] Synchronization failed with the following exception: Object reference not set to an instance of an object
  at (wrapper managed-to-native) Gtk.ListStore:gtk_list_store_set_value (intptr,intptr,int,intptr)
  at Gtk.ListStore.SetValue (TreeIter iter, Int32 column, Value value) [0x00000] in <filename unknown>:0 
  at Gtk.ListStore.AppendValues (System.Array values) [0x00000] in <filename unknown>:0 
  at Gtk.ListStore.AppendValues (System.Object[] values) [0x00000] in <filename unknown>:0 
  at Tomboy.NoteRecentChanges.UpdateResults () [0x00000] in <filename unknown>:0 
  at Tomboy.NoteRecentChanges.OnNoteSaved (Tomboy.Note note) [0x00000] in <filename unknown>:0 
  at (wrapper delegate-invoke) <Module>:invoke_void__this___Note (Tomboy.Note)
  at Tomboy.NoteManager.OnNoteSave (Tomboy.Note note) [0x00000] in <filename unknown>:0 
  at Tomboy.Note.Save () [0x00000] in <filename unknown>:0 
  at Tomboy.Sync.SyncManager.SynchronizationThread () [0x00000] in <filename unknown>:0 
[DEBUG 15:22:15.582] SilentUI: SyncStateChanged: Idle
  at (wrapper managed-to-native) Gtk.Application.gtk_main () <IL 0x0000e, 0xffffffff>
  at Gtk.Application.Run () <IL 0x00000, 0x0000b>
  at Tomboy.GnomeApplication.StartMainLoop () <IL 0x00000, 0x0000b>
  at Tomboy.Application.StartMainLoop () <IL 0x00005, 0x00022>
  at Tomboy.Tomboy.StartTrayIcon () <IL 0x00014, 0x00053>
  at Tomboy.Tomboy.Main (string[]) <IL 0x0014a, 0x0031b>
  at (wrapper runtime-invoke) <Module>.runtime_invoke_void_object (object,intptr,intptr,intptr) <IL 0x00050, 0xffffffff>

Native stacktrace:

	mono() [0x4911ea]
	mono() [0x4e064f]
	mono() [0x41b187]
	/lib/x86_64-linux-gnu/libpthread.so.0(+0x10060) [0x7f35ee2e1060]
	/lib/x86_64-linux-gnu/libc.so.6(+0x1343c0) [0x7f35ee0663c0]
	/lib/x86_64-linux-gnu/libglib-2.0.so.0(g_array_insert_vals+0x59) [0x7f35eb70e549]
	/usr/lib/x86_64-linux-gnu/libgtk-x11-2.0.so.0(+0x21e585) [0x7f35eb043585]
	/usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0(g_closure_invoke+0xaa) [0x7f35e90f9ffa]
	/usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0(+0x2102a) [0x7f35e910c02a]
	/usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0(g_signal_emit_valist+0x851) [0x7f35e91156b1]
	/usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0(g_signal_emit+0x82) [0x7f35e9115852]
	/usr/lib/x86_64-linux-gnu/libgtk-x11-2.0.so.0(+0x21a514) [0x7f35eb03f514]
	/usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0(g_closure_invoke+0xaa) [0x7f35e90f9ffa]
	/usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0(+0x2102a) [0x7f35e910c02a]
	/usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0(g_signal_emit_valist+0x851) [0x7f35e91156b1]
	/usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0(g_signal_emit+0x82) [0x7f35e9115852]
	/usr/lib/x86_64-linux-gnu/libgtk-x11-2.0.so.0(gtk_list_store_set_value+0x13c) [0x7f35eaf5760c]
	[0x40c4e50d]

Debug info from gdb:

Could not attach to process.  If your uid matches the uid of the target
proces[DEBUG 15:21:05.254] Saving 'Ubuntu Oneiric Ocelot: Post-install'...
[DEBUG 15:21:05.390] Note saved or deleted within a minute of next autosync...resetting sync timer
[DEBUG 15:21:05.857] Saving 'To Do: Some Day'...
[DEBUG 15:21:05.896] Note saved or deleted within a minute of next autosync...resetting sync timer
[DEBUG 15:22:05.898] BackgroundSyncChecker: Checking server for updates
[DEBUG 15:22:05.899] Building web request for URL: https://one.ubuntu.com/notes/api/1.0/
[DEBUG 15:22:08.903] Building web request for URL: https://one.ubuntu.com/notes/api/1.0/user/
[DEBUG 15:22:10.084] BackgroundSyncChecker: Detected that sync would be a good idea now
[DEBUG 15:22:10.084] SyncThread using SyncServiceAddin: Tomboy Web
[DEBUG 15:22:10.085] SilentUI: SyncStateChanged: Connecting
[DEBUG 15:22:10.130] SilentUI: SyncStateChanged: AcquiringLock
[DEBUG 15:22:10.131] Building web request for URL: https://one.ubuntu.com/notes/api/1.0/
[DEBUG 15:22:10.444] Building web request for URL: https://one.ubuntu.com/notes/api/1.0/user/
[DEBUG 15:22:11.370] 8
[DEBUG 15:22:11.370] SilentUI: SyncStateChanged: PrepareDownload
[DEBUG 15:22:11.370] Sync: GetNoteUpdatesSince rev 758
[DEBUG 15:22:11.370] Building web request for URL: https://one.ubuntu.com/notes/api/1.0/op/?include_notes=true&since=758
[DEBUG 15:22:13.144] Sync: 0 updates since rev 758
[DEBUG 15:22:13.144] Building web request for URL: https://one.ubuntu.com/notes/api/1.0/op/
[DEBUG 15:22:15.511] SilentUI: SyncStateChanged: PrepareUpload
[DEBUG 15:22:15.511] Saving 'Ubuntu Oneiric Ocelot: Post-install'...
[DEBUG 15:22:15.516] Saving 'Ubuntu Oneiric Ocelot: Post-install'...

(Tomboy:22598): Gtk-CRITICAL **: IA__gtk_tree_model_row_inserted: assertion `path != NULL' failed
Stacktrace:

  at (wrapper managed-to-native) Gtk.ListStore.gtk_list_store_set_value (intptr,intptr,int,intptr) <IL 0x00012, 0xffffffff>
  at Gtk.ListStore.SetValue (Gtk.TreeIter,int,GLib.Value) <IL 0x00021, 0x0009f>
  at Gtk.ListStore.AppendValues (System.Array) <IL 0x0002f, 0x0010b>
  at Gtk.ListStore.AppendValues (object[]) <IL 0x00002, 0x00027>
  at Tomboy.NoteRecentChanges.UpdateResults () <IL 0x000f8, 0x003bf>
  at Tomboy.NoteRecentChanges.OnNoteSaved (Tomboy.Note) <IL 0x00013, 0x0003f>
  at (wrapper delegate-invoke) <Module>.invoke_void__this___Note (Tomboy.Note) <IL 0x00056, 0xffffffff>
  at Tomboy.NoteManager.OnNoteSave (Tomboy.Note) <IL 0x00012, 0x0002c>
  at Tomboy.Note.Save () <IL 0x000f0, 0x00244>
  at Tomboy.Note.SaveTimeout (object,System.EventArgs) <IL 0x00001, 0x0001f>
  at Tomboy.InterruptableTimeout.TimeoutExpired () <IL 0x00018, 0x00029>
  at GLib.Timeout/TimeoutProxy.Handler () <IL 0x0000d, 0x0003a>
  at (wrapper native-to-managed) GLib.Timeout/TimeoutProxy.Handler () <IL 0x00029, 0xffffffff>
[ERROR 15:22:15.582] Synchronization failed with the following exception: Object reference not set to an instance of an object
  at (wrapper managed-to-native) Gtk.ListStore:gtk_list_store_set_value (intptr,intptr,int,intptr)
  at Gtk.ListStore.SetValue (TreeIter iter, Int32 column, Value value) [0x00000] in <filename unknown>:0 
  at Gtk.ListStore.AppendValues (System.Array values) [0x00000] in <filename unknown>:0 
  at Gtk.ListStore.AppendValues (System.Object[] values) [0x00000] in <filename unknown>:0 
  at Tomboy.NoteRecentChanges.UpdateResults () [0x00000] in <filename unknown>:0 
  at Tomboy.NoteRecentChanges.OnNoteSaved (Tomboy.Note note) [0x00000] in <filename unknown>:0 
  at (wrapper delegate-invoke) <Module>:invoke_void__this___Note (Tomboy.Note)
  at Tomboy.NoteManager.OnNoteSave (Tomboy.Note note) [0x00000] in <filename unknown>:0 
  at Tomboy.Note.Save () [0x00000] in <filename unknown>:0 
  at Tomboy.Sync.SyncManager.SynchronizationThread () [0x00000] in <filename unknown>:0 
[DEBUG 15:22:15.582] SilentUI: SyncStateChanged: Idle
  at (wrapper managed-to-native) Gtk.Application.gtk_main () <IL 0x0000e, 0xffffffff>
  at Gtk.Application.Run () <IL 0x00000, 0x0000b>
  at Tomboy.GnomeApplication.StartMainLoop () <IL 0x00000, 0x0000b>
  at Tomboy.Application.StartMainLoop () <IL 0x00005, 0x00022>
  at Tomboy.Tomboy.StartTrayIcon () <IL 0x00014, 0x00053>
  at Tomboy.Tomboy.Main (string[]) <IL 0x0014a, 0x0031b>
  at (wrapper runtime-invoke) <Module>.runtime_invoke_void_object (object,intptr,intptr,intptr) <IL 0x00050, 0xffffffff>

Native stacktrace:

	mono() [0x4911ea]
	mono() [0x4e064f]
	mono() [0x41b187]
	/lib/x86_64-linux-gnu/libpthread.so.0(+0x10060) [0x7f35ee2e1060]
	/lib/x86_64-linux-gnu/libc.so.6(+0x1343c0) [0x7f35ee0663c0]
	/lib/x86_64-linux-gnu/libglib-2.0.so.0(g_array_insert_vals+0x59) [0x7f35eb70e549]
	/usr/lib/x86_64-linux-gnu/libgtk-x11-2.0.so.0(+0x21e585) [0x7f35eb043585]
	/usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0(g_closure_invoke+0xaa) [0x7f35e90f9ffa]
	/usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0(+0x2102a) [0x7f35e910c02a]
	/usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0(g_signal_emit_valist+0x851) [0x7f35e91156b1]
	/usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0(g_signal_emit+0x82) [0x7f35e9115852]
	/usr/lib/x86_64-linux-gnu/libgtk-x11-2.0.so.0(+0x21a514) [0x7f35eb03f514]
	/usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0(g_closure_invoke+0xaa) [0x7f35e90f9ffa]
	/usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0(+0x2102a) [0x7f35e910c02a]
	/usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0(g_signal_emit_valist+0x851) [0x7f35e91156b1]
	/usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0(g_signal_emit+0x82) [0x7f35e9115852]
	/usr/lib/x86_64-linux-gnu/libgtk-x11-2.0.so.0(gtk_list_store_set_value+0x13c) [0x7f35eaf5760c]
	[0x40c4e50d]

Debug info from gdb:

Could not attach to process.  If your uid matches the uid of the target
process, check the setting of /proc/sys/kernel/yama/ptrace_scope, or try
again as the root user.  For more details, see /etc/sysctl.d/10-ptrace.conf
ptrace: Operation not permitted.
No threads.

=================================================================
Got a SIGSEGV while executing native code. This usually indicates
a fatal error in the mono runtime or one of the native libraries 
used by your application.
=================================================================

Aborted
s, check the setting of /proc/sys/kernel/yama/ptrace_scope, or try
again as the root user.  For more details, see /etc/sysctl.d/10-ptrace.conf
ptrace: Operation not permitted.
No threads.

=================================================================
Got a SIGSEGV while executing native code. This usually indicates
a fatal error in the mono runtime or one of the native libraries 
used by your application.
=================================================================

Aborted