Basically, start seahorse and delete all Ubuntu One related stored passwords (perhaps not all is needed, but I deleted all). Reconnect. You will be asked for usename/password.
Hopefully, this helps for some others who still are having problems.
It would have been good to know what caused the problem, because I have not had made any changes when the problem had occurred.
Following the instructions in the following page solved my "auhentication" problem in Ubuntu One.
https:/ /one.ubuntu. com/help/ faq/what. .._failed- state/
Basically, start seahorse and delete all Ubuntu One related stored passwords (perhaps not all is needed, but I deleted all). Reconnect. You will be asked for usename/password.
Hopefully, this helps for some others who still are having problems.
It would have been good to know what caused the problem, because I have not had made any changes when the problem had occurred.
Cheers.
tuuk