Comment 2 for bug 1594841

There are two completely separate issues here:

1) The webapp authentication problem is real, and works exactly as you described. It's a trick to improve the first-time experience of the webapps, and it's not reliable in any way. However, let's discuss it in a separate bug report, please (we probably already have some bugs about that).

2) The Youtube authentication problem could be caused by the Youtube scope not properly handling the error from the remote server: youtube receive a cached token from OA, which most likely is getting refused by the server, but the scope doesn't handle the case properly.
The proper handling of this error would be invalidating the existing token (the OAuth plugin has a special ForceTokenRefresh option for this case).

I don't see how this is related to UbuntuOne in any way.