I really don't know what to think.

I tried your advise, but it seems there is nothing to do: it still does not prints.
I tried even connecting the printer via the parallel port (just another try...) but - obviously - it wasn't useful. Of course, the printer is fully functional, as I dual boot with windows, and in windows I can print.

I tried to inspect the printing queue; there are only the test print jobs, spooled and terminated, not pending. Well, looking at the IPP log I can see, on the last row:

"job-state-reason: aborted-by-system"

but why??