Make sure you have all the latest updates on your computer. Open System->Administration->Update Manager, click the "Check" button, and if there are updates to install, click the "Install Updates" button.
Once all the updates are installed, follow these steps:
1、Open System->Preferences->Passwords and Encryption Keys
2、Under "Passwords", right-click on Ubuntu One and select Delete
3、Open Applications->Accessories->Terminal and run:
killall ubuntu-sso-login
u1sdtool -q; u1sdtool -c
That command should cause the Ubuntu One SSO dialog to appear. From there you can either sign up for a new Ubuntu One account or add your computer to an existing Ubuntu One account.