“Your migration failed checking source credentials. Error connecting to target SharePoint site, please retry after a while: <Site URL> Error: Cannot Retrieve List: Access denied. You do not have permission to perform this action or access this resource.: Shared Documents”
This error is caused by an intermittent SharePoint bug within the Microsoft system. In most cases, this issue will resolve itself after a few minutes.
Wait at least 10 minutes, then try the migration again.