Synchronization state data is corrupt or otherwise invalid

If you get an Unhandled Exception in your mailbox profile alerts, but no real information, where it comes from, then this post will help you.

I got the following alert in my mailbox:

<params>
	<param type="0" otc="9606" oid="48d028d3-8c53-e811-9103-005056a26140">Mittermair, Michael</param>
	<param type="0" otc="9605" oid="9dd1ce55-e851-e811-9102-005056a26140">MailProfile</param>
</params>

<details>
	<detail key="servererrorcode">Exchange.server returned ErrorInvalidSyncStateData error.</detail>
	<detail key="servererrorhelplink">Microsoft.Crm.Asynchronous.EmailConnector.ErrorSource.ExchangeError:ErrorInvalidSyncStateData</detail>
</details>	1	NULL	BACKEND	ExchangeSyncUnknownACTIntegrationError	0	T:109
ActivityId: 8820ea97-0688-4cff-b9c7-b1fc8da070d6

I enabled the tracing on the backend server and also checked the log in the database and found the following:

>Exception : Unhandled Exception: Microsoft.Exchange.WebServices.Data.ServiceResponseException: Synchronization state data is corrupt or otherwise invalid.    
at Microsoft.Exchange.WebServices.Data.ServiceResponse.InternalThrowIfNecessary()    
at Microsoft.Exchange.WebServices.Data.MultiResponseServiceRequest`1.EndExecute(IAsyncResult asyncResult)   
at Microsoft.Exchange.WebServices.Data.ExchangeService.EndSyncFolderHierarchy(IAsyncResult asyncResult)    
at Microsoft.Crm.Asynchronous.EmailConnector.MonitoredExchangeService.EndSyncFolderHierarchy(IAsyncResult asyncResult)    
at Microsoft.Crm.Asynchronous.EmailConnector.ExchangeSyncSteps.ExchangeSyncAsyncRemoteStep`2.AfterCall()    
at Microsoft.Crm.Asynchronous.EmailConnector.ExchangeSyncSteps.ExchangeSyncAsyncRemoteStep`2.Execute()    
at Microsoft.Crm.Asynchronous.EmailConnector.ExchangeSyncWorker.SynchronizeACTs()

Based on the first error message, I could elicit, that it is connected to an activity. And that it’s connected to the ACT-Settings in my Sync-Profile. Therefore I checked the settings which looked like the following:

• A specific user for synchronization. This user has impersonation-rights on the mailboxes of the crm users.

Based on the Microsoft documentation https://docs.microsoft.com/en-us/dynamics365/customer-engagement/admin/best-practices-server-side-synchronization there is a hint, that ACT is not supported with Mailbox Impersonation.

Therefore I changed my profile to enter my own credentials for the mailbox and the error disappeared and the sync worked again for ACT.