After several random tests, I found what prevents printing and displays
the error message "filter failed" in the web interface of cups: 

The ppd file I used uses the postscript language, but if I use the ppd
file in PDF language, printing works:

Ricoh-MP_C3003-PDF-Ricoh.ppd: "ok"
Ricoh-MP_C3003-Postscript-Ricoh.ppd: "filter failed"

Since openprinting offers both ppd files, that's okay, but I would find
it useful to know if there is anything to specify in the guix
configuration file for postscript ppd to work.

Attachment: config.scm