Have you double checked your permissions on the files? Usually, when you upload the files via your account FTP user, they do not get uploaded with the same permissions as what you need.
In my case, I first checked to see who owned the files, and it turned out to be my user account logon. However, when Joomla is executing and modifying files, it does it as Nobody. It may be different on your system, or a different user. So, I changed all my files to be owned by Nobody, and then CHMODed them to 644.
As soon as I did that, I no longer got the "Not Authorized" error, and a number of other things worked much better as well.
I hope this helps.