Comment 12 for bug 36532

You seem to be missing the /etc/cups/pstoraster.convs file which is part of gs-esp.

Please give the output of the following command:
$ dpkg -L gs-esp | grep pstoraster

Then reinstall gs-esp with following command and afterwards check if the file is present:
$ sudo apt-get --reinstall install gs-esp

Restart cups:
$ sudo /etc/init.d/cupsys restart

Reinstall your printer with the original ppd for your printer that came with Ubuntu.

Try to print a postscript and text file from the command line:
$ lp file.ps
$ lp file.txt