What version of the zimbra server are you running against?
The migration wizard always authenticates with the admin account (the information provided in the second step of the wizard).
If you are able to get past the second screen, the authentication should succeed during the import phase.
Is the target account provisioned? Did you provision it with the migration wizard? Was the account disabled by accident?
You can see why the auth failed by enabling the WinHttp tracing to see the actual soap request/response. See this post on how to enable WinHttp tracing.
Remove any sensitive information from the WinHttp trace file and post it here (or just the response to the AuthRequest that failed).