Thank yoy for your interest and answers.
Platonides:
So you shouldn't have tried to convert the content, that would have broken it.
Very likely this is what they did.
Without knowing the exact way your db is broken, it'd be hard to fix it. What search are you using? The internal one? Maybe you can share a one-revision sql dump?
And your crystall ball? ;-) Well, as to the problem with the search I've discovered the key for the searchindex table is generated with U8<hex> while the table has u8<hex>. So, if I replace in SeachMySQL.php
$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?).
PS: Update to 1.15.2, you are vulnerable to CSS image inclusion.
Thank you. Please, could you give a pointer explaining it?
Cheers Javier