I wrote:
$searchon = $this->db->strencode( $searchon );
by
$searchon = $this->db->strencode( $wgContLang->lc ($searchon) );
words are found. I'm using this as a workaround, but I'm investigating
the origin of the problem (any hints?).
Found. The problem is in the database. For some reason they have
decided the table searchindex should have a collation using latin1 :-(.
So I repeat my second question:
2) In the medium term, and with the background above,
which
is the best approach to have a "correct" database and system
again.
Cheers
Javier