Hello
DSA keys have been added in 5.5.1 but it seems you have tried with it.
You succeed with 4.0.1 because fingerprint verification of the source machine is not done, which is less secure. The verification has been added in 5.1, i.e the behavior you get when logging in the helper VM is expected.
You could use 5.0.1 as workaround, too. You can convert to vSphere 5.5 with it, it will just create a lower VM hardware version. Don't try with 5.0 though, it will crash when targeting vSphere version higher than 5.0!
However this looks like a bug that should be fixed. Can you please upload the logs for investigation? Export the full bundle from the failed task and attach it here (you will need to click on 'Use advanced editor' on the forum's page)
Regards,
Plamen