After upgrading from 13.10 to 14.04 and knowing that gutenprint 5.2.10 now supports Canon SELPHY printers, I've tried installing my CP400 through Settings > Printers.
It finds the printer fine and lists the URI as:
...and then appears to find the drivers. But when I hit the apply button, I get:
CUPS Server Error: There was an error during the CUPS operation: 'client-error-not-possible'.
If I go to localhost:631/Admin and view the error log, the error appears as:
Returning IPP client-error-not-possible for CUPS-Add-Modify-Printer (ipp://localhost/printers/Canon-CP400) from localhost
Note that the path in the error is different than the path as entered. Also, it seems that "gutenprint52usb" should have a plus sign in it - gutenprint52+usb - since that would then match the actual file name. I did try changing "gutenprint52usb" to "ipp", per some answers I found in my search. That added the printer just fine, but of course, it didn't work (as in nothing got to the printer).
I've tried reinstalling both CUPS and gutenprint through Synaptic. Rebooted. Plugged/unplugged the printer numerous times. Etc.
If instead I attempt to add the printer through the localhost CUPS utility, it asks for a username and password. I only have one password on this machine and it works every time I log on or need to use sudo, so seems like it should work here but, alas, no. Nor does using my username or root and the same password.