If you were using the beta version of Ubuntu 9.10 karmic koala you might have problems with your Ubuntu one client. After you upgrade your system to the final release, you may still have a problem.
Here is how I fixed it in my system. I opened synaptic package manager and noticed I had both a ubuntuone-client and a ubuntuone-client-beta. The beta was the one installed, so I chose the one that didn’t have the beta. In the process of installing it will remove the beta version. After that I then right clicked on the Ubuntu One icon on the upper task bar and closed it. I then went to Applications > Internet and clicked on Ubuntu One to restart it. It seems to be working fine now.