com.sun.jersey.api.client.ClientHandlerException from UPM

I'm encountering "An error occurred while connecting to the Atlassian Plugin Exchange. For more information about this problem, please refer to the UPM documentation" error on a machine that can access http://plugins.atlassian.com directly.

The catalina logs report exception com.sun.jersey.api.client.ClientHandlerException "Unbuffered entity enclosing request can not be repeated.", (see attached log).