I was using my Epson Stylus Photo R1800 successfully with GIMP before I upgraded from 10.04 to 12.04. Since then, prints of my photographs are pale and unsharp in comparison to the screen.
Trying to change the driver, I get the message I have given in the Title of this post, with the following text: "This action is not supported by this backend. Please report a bug as this should not have happened. Provides postscript-driver not supported."
A screenshot of the message is here: