In includes/sessions.php file,
find:
$username = (isset($row[$user_table_fields['user_level']]) && $row[$user_table_fields['user_level']] == ADMIN && $config['highlight_admin'] == 1) ? sprintf("<b>%s</b>", $row[$user_table_fields['user_name']]) : $row[$user_table_fields['user_name']];
replace:
$username = (isset($row[$user_table_fields['user_level']]) && $row[$user_table_fields['user_level']] == ADMIN && $config['highlight_admin'] == 1) ? sprintf("<b>%s</b>", format_text(trim($row[$user_table_fields['user_name']]), 2)) : format_text(trim($row[$user_table_fields['user_name']]), 2);