it looks to me like your site is trying to insert text in utf-8 format into a latin1 db.
Check the format of your db. for phpBB2 it should be latin1_* , probably latin1_swedish_ci
delete the search_wordlist table and reinstall it
#
# Table structure for table `phpbb_search_wordlist`
#
CREATE TABLE phpbb_search_wordlist (
word_text varchar(50) binary NOT NULL default '',
word_id mediumint(8) unsigned NOT NULL auto_increment,
word_common tinyint(1) unsigned NOT NULL default '0',
PRIMARY KEY (word_text),
KEY word_id (word_id)
) TYPE=MyISAM;
"Success is getting what you want. Happiness is wanting what you get." - Dale Carnegie