Hi Klaus, I found the problem
I don't know why, but the Default.ly in OOoLilyPond-templates directory ,
that's located in my home directory, /home/dario/, had no "\include
lilypond-preamble.ly"
line; I think because may be it was coming from prevoius version of the
extension; after deleting every file and directory in my home referring to
OOoLilypond, I removed the extension and I added it again, and now it works
properly.
May be it would have been enough to do restore default templates, but I
didn't think to do it.