DrByte seems to have a fix. See thread
http://www.zen-cart.com/showthread.p...highlight=migs
I have followed Dr Bytes advice and tested it.
I am using a TEST merchant account.
I entered invalid cc details on the Commweb Server Managed Payments and was returned to my test cart with an error.
I then repeated checkout with then valid cc details on the Commweb Server Managed Payments and successfully completed the transaction and was returned to my test cart with a successful order.
So, it works for me.
Version 1.5.0, CBA, VPC, Server Managed Payments, Test Host Simulator.
BTW had a problem with site hanging and going to a blank page when going from checkout_payment to checkout_confirmation. Fixed by selecting "none" in the "payment zone" of cc_via_migs settings. May be a problem in cc_via_migs.php where it checks zone.