EWS recurring errors (Calender only)

Hi,

I’m writing in this forum for the first time and I hope it will be the right platform to get answers to my problem.  The company I work for has developed an interface using Exchange WebServices.  It works pretty well, but one of our customer faces the following problems:

1. Anyone knows what this means: Synchronization state data is corrupt or otherwise invalid.
2. Why other kind of errors ( The remote server returned an error: (401) Unauthorizedor The folder to be synchronized could not be found.) do not occur regularly (sometime the webservice returns an error and other rimes it does not)

Not all Exchange Accounts face these problems, and an Account with a problem (401) does sometimes work. Could it depend on the specific settings of the account or on the general settings of the server?

Thanks!