Can you please try the following:
1. Quit the Ubuntu One Preferences, if open
2. Open (Lucid): Applications->Accessories->Passwords and Encryption Keys
(Maverick): System -> Preferences -> Password and Encryption Keys
3. Click on the arrow next to "Passwords"
4. Right-click on the Ubuntu One token and select "Delete"
5. Go to https://one.ubuntu.com/account/machines/
6. Click on the checkbox next to your computer
7. Click the "Remove selected computers" button
8. (Maverick): killall ubuntu-sso-login; u1sdtool -q; u1sdtool -c
(Lucid): u1sdtool -q; killall ubuntuone-login; u1sdtool -c
9. a web page, if in Lucid, or a window, in Maverick, should open,prompting you to add your computer to your Ubuntu One account
10. Add your computer