By default, UbuntuOne does not work behind a proxy. To make it work, you have to install ubuntuone-client-proxy (sudo apt-get install -y ubuntuone-client-proxy ), then restart the UbuntuOne. Now there is one more ubuntuone related process running in background.
ps aux | grep ubuntuone
jchen 8688 2.7 1.4 180816 56852 ? Sl 13:20 0:01 /usr/bin/python /usr/bin/ubuntuone-control-panel-qt
jchen 8707 5.0 0.7 112780 30348 ? Sl 13:20 0:02 /usr/bin/python /usr/lib/ubuntuone-client/ubuntuone-syncdaemon
jchen 8737 0.5 0.5 67576 19980 ? S 13:20 0:00 /usr/bin/python /usr/lib/ubuntuone-client/ubuntuone-proxy-tunnel fs-1.one.ubuntu.com 443
Jackie Chen's IT Workshop 