Comment 11 for bug 959451

Thanks a lot Samuel, your screen shots have helped me get to the bottom of the issue.

Credentials from the keyring are not inserted by the gtk dialog, but by another process (system-config-printer-applet) after the print dialog has sent the job to the server. CUPS sets AuthInfoRequired when authentication fails for some reason (maybe the printer was unreachable or the password changed). From this point on, the print dialog thinks it needs to ask for credentials, because it doesn't know that system-config-printer will insert them later. Hence, simply clicking OK without entering the password works.

The fix will be to have the gtk dialog read credentials from the keyring. I'm assigning myself to do this, but it's too late in the cycle to get this into 12.04.

I'm afraid the only workarounds -- short of patching CUPS or gtk -- are removing the AuthInfoRequired line manually every time it is changed or simply confirming the password prompt with empty user name and password.