Migration fails with error: java.lang.Exception: Server Response (4239158)

Migrating objects (from SharePoint Online to SharePoint Online) using the Navigator Pane, it fails with error:

OAuth is selected under "Hyper Migration Settings" and the Migration Client is not installed in the tenants (Source and Target)

Recommended: Configure OAuth Framework Authentication following the steps here: OAuth Framework Authentication

Or

You could switch back to the "Classic Navigator Mode", by deactivating OAuth and Activating "Classic Navigator Mode"

Note: Basic authentication is heavily throttled and may be blocked in some tenants.