One day I had a problem loging on with ubuntu one on a Ubuntu 10.04 machine. It logged on, but didn't synchronise. I mailed the ubuntu one helpdesk and these guys were very helpfull.
The asked me to send them the logfiles and discovered two problems. First the internal clock must be right. If your using daylight saving in winter it goes wrong. After that was corrected, The problem persisted only in one account on that specific computer. On other computers that account worked very well and on this specific computer other accounts worked OK as well. As you can see it logged on, but didn't sync (onbekend means unknown)
They suggested me the following:
Start a terminal or press Alt-F2, type "gnome-terminal", press "Enter" and run:
sudo rm -rf ~/.local/share/ubuntuone
This stopped the ubuntu one deamon, removed some files in ~./local/shera/ubuntuone and restarts the deamon.
After restarting the syncing started at once and I was happy again.
Once again, the credit goos to the guys at the helpdesk of ubuntu one. A great job!