Allegedly, on or about 01 June 2014, Mike Wright sent:
Cups isn't being very helpful here either. Almost anything administrator related tells me "Forbidden", but nowhere can I find a way to "Login" to it.
In past times, as soon as you tried to do something that required privileges, the CUPS webpage would prompt you to enter them, with the standard gadget that the web browser used for entering username and passwords. They were the root username and password. I don't know if they've changed it to require your username and password.
But, otherwise, look in /var/log/cups/ for an error log, and read it.