Comment 2 for bug 530079

Karmic release note:

Ubuntu One files may become unavailable in some circumstances

A bug in the ubuntu one client (#462828) can cause files to become unavailable in some circumstances. A new client has been released in the update channel and the server has been updated to show a "Capabilities Mismatch" error when in use with a an older client to prevent files becoming unavailable to users.

Manually running an system update will update the client to work with the deployed server and fix the root cause. Files are still available via the web interface at http://one.ubuntu.com