[naisho]

The solution given by nektar worked fine for me.
In functions_bop_wordlinks.php file, I did that (around line 121):

//    $bopfind = preg_quote (htmlspecialchars($bopfind));
      define('CHARSET', 'ISO-8859-1');
      define('REPLACE_FLAGS', ENT_COMPAT | ENT_XHTML);  
      $bopfind = preg_quote (htmlspecialchars($bopfind,REPLACE_FLAGS, CHARSET));