The most common cause for this issue is the URL set incorrectly for the migration. Try removing the /OWA as sometimes Outlook web access has been configured for access on the root directory.

This error can also occur if the credentials used to access the mailbox configured are invalid, or the credentials specified do not have access to to the configured mailbox, or you may be have over-utilised resources.  You can verify that the user name and password is correct by testing the access using webmail . If you are using admin level access, make sure to log in to the mailbox using administrator credentials and then try connecting to the mailbox required using the option within webmail to select another mailbox.

Poor resource can also cause this issue. In this case please check the following:-

  1. Server CPU usage
  2. Memory usage
  3. Disk I/O usage using performance monitor
  4. Over-utilisation of the network.

If migrating multiple concurrent mailboxes try reducing the number you run at the same time.