**** Warning: considering '0000000000 XXXXX n' as a free entry.https://codeyarns.com/2014/03/10/free-entry-warning-with-ghostscript-and-imagemagick/
This warning turns out to be from the Ghostscript program, which is used by ImageMagick internally to perform this format conversion. This page explains that this error has been fixed in a recent version of Ghostscript. I used a newer version of Ubuntu, that is 13.11, that had a newer version of gs and I was able to perform the format conversion. It still gave off the warning, but the conversion was done.
Image.py tells imagemagick to convert to stdout. Warnings are also returned via stdout before actual rendering.
I also tested with the previous imagemagick used on Nexedi ERP5, the issue still happens.
So far, I have no other quick solution except by testing latest ghostscript + imagemagick (for instance on a Debian or Ubuntu machine before trying upgrade them in SlapOS).