It's possible that ActiveSync has an internal problem with its synchronization table.
You may try the following operations:
- Delete you ActiveSync partnership on both the desktop ("ActiveSync/Tools/Delete mobile device") and the mobile (ActiveSync/Menu/Options)
- After having made a backup, delete the following directory:
XP: C:\Documents and Setting\\Applications Data\Microsoft\Activesync\Profiles\
- Reconnect your mobile and configure a new synchronization