After one week without more explanations about the replacements for iso-8859-1 coded sites I found a "dirty" way to make it work. I have added some lines to /components/com_sef/joomsef.php file using the joomlaxplorer component (extensions.joomla.org/component/option,c...nk_id,102/Itemid,35/).

After line 678 [// Replace non-ASCII characters.] i have added one line for every character to replace.

$title = str_replace("ñ", "n", $title);
$title = str_replace("á", "a", $title);
$title = str_replace("é", "e", $title);
$title = str_replace("í", "i", $title);
$title = str_replace("ó", "o", $title);
$title = str_replace("ú", "u", $title);

Perhaps it is not the best solution but it works...

The sobi2 plugin continues working without the correct replacement, there is some possibility of a refund for the plugin I have paid?