Received an empty WebDAV response: The Exchange server issued an empty XML response when responding to an HTTP WebDAV request. This may happen if the Exchange server is under significant stress, or if the account specified has insufficient permissions on the mailbox.
Check if the Exchange server is under stress. Some reasons may include: an archiving agent putting an unexpected load on the server, a large number of concurrent migrations are opening many connections, poor hardware specifications, etc.
We also recommend verifying that the specified account has permissions to log in to the specified mailbox using the following instructions.