As so often happens, just after you post to a forum, and walk over to get coffee, you have a sudden brainwave and find the answer to your own problems! :-)
There was an orphan empty .xml file in the applications folder. asupgrade tried to make the file a .jar of 0 bytes in size, which it places in /tmp for the deployment phase into 2.1 - hence the exception.
The migration runs smoothly after removing the file.
Craig
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