Response status code does not indicate success: 500 (4379521)

After matching the accounts, Mail Migration Task gave these errors:

Response status code does not indicate success: 500 (Internal Server Error). (Response status code does not indicate success: 500 (Internal Server Error).)

and

Error creating source session. Response status code does not indicate success: 500 (Internal Server Error).(Response status code does not indicate success: 500 (Internal Server Error).)

Consent details not updated properly: 

In this case: "Mailbox Migration - Custom RBAC (exchangeCustomRBAC)" consent has been granted first, and then later it has been revoked

Perform a full refresh of the page, or alternatively navigate out of the project and re-enter the project which will perform a full re-check of consents.