fwiw: I can recreate this at will... The setup: the jenkins-cli jarfile with Jenkins running in Winstone/Jetty/Tomcat/JBoss/Doesn'tMatter and Apache frontending Jenkins with a ProxyPass.
Trying to access Jenkins thru Apache via: java jenkins-cli.jar -s http://apache.example.com/ will cause this to happen. The CLI tries to setup a full duplex link thru apache and when creating the upload channel, apache will fail with the EAGAIN. If using haproxy as a front end, the cli works... it ignores the initial timeout/eagain on the socket and then goes ahead and reads... you do see the delay, but after the delay, the connection goes thru.
