Comment 1 for bug 1094184

Can you try the following steps?

1. If it is open, quit Ubuntu One's control panel by right clicking on the tray icon and choosing "Quit"
2. Hit Win+W and then type "credential"
3. Select "Manage Windows Credentials"
4. In the "Generic Credentials" section, click on any items that have Ubuntu One in their name.
5. Choose the "Remove" option.

At that point you can restart Ubuntu One, provide your username and password again, and it should work.

That's a template I've been sending a few people - it works for some, doesn't work for others. We're still trying to reproduce this issue and figure it out.