http://ariejan.net/2006/11/13/cups-426-upgrade-required/

After some research I came to the conclusion that CUPS, by default, tries to use SSL whenever possible. So, with this 426 error, you are redirected to the SSL domain. Chances are, you haven’t configured SSL properly, if at all.

In my case, I didn’t want to configure SSL. To get rid of this problem, the key lies in editing your configuration files ( /etc/cups/cupsd.conf ) and adding the following line:

DefaultEncryption Never

There are several options, Never, IfRequired and Required. By setting this to Never, SSL will never be enforced. Just restart your CUPS server with

$ /etc/init.d/cupsys restart