The element at position 0 is invalid: This error indicates that Exchange Web Services (EWS) returned an invalid SOAP response. More specifically, we submitted a SOAP request to an Exchange Server so as retrieve a batch of items to migrate. The Exchange Server returned an invalid SOAP response, including an invalid element at position 0 (i.e., an empty element, an element without an ID, etc.).
This error is not fatal and will normally not interrupt the migration. This error is often resolved by retrying failed items once the migration completes. You may also want to check server load (CPU load, memory load, IO load) at the Source. This error is likely to occur more frequently when connecting to over-utilized Exchange systems/Exchange systems under stress.