From the log files you can see the following error message:
“Synchronization state data is corrupt or otherwise invalid.”
There are also less severe error messages:
“firstname.lastname@example.org not found in configuration”
This usually occurs when a user was set up in the initial configuration but has since been removed from the sync.
The best thing to do in this case, especially because there is some invalid sync state data would be to reset the exchange sync.
To do this you basically delete the existing exchange integration and set it up again. This clears down all the sync state information in the Ecng and EWS sync tables and recreates them again when the new sync is set up.
There are de-duplication rules written into the code that will prevent duplicate appointments from being created in CRM or exchange once the new sync has been set up.