I can not clean the HP 6988 printhead from Fedora 13

[Fabrizio Celentano]

  I needed to clean the print head of my HP Deskjet 6988dt printer, which works pretty well under Fedora 13. The driver automatically selected on installation performs all the requested operations.
  Only print head cleaning (system administration>printing>properties>clean button) does not work, and I was not able to find a way to perform the operation form the front panel. The error message does not say anything to me:

CUPS server error. There vas an error during the CUPS operation:'client-error-document-format-not-supported"

The old nov. 2007 trick (sudo hp-toolbox), http://free-linux-trick.blogspot.com/2007/11/head-cleaning-hp-printer-all-linux.html does not work.
Someone can help? Thanks to all.
fcc