I've got all settings on default for the search and when I wanted to search for the word "bla" I got this:

General Error
SQL ERROR [ mysql ]

Every derived table must have its own alias [1248]

SQL

SELECT COUNT(post_id) as total_results FROM (SELECT DISTINCT p.post_id FROM (phpbb2_search_wordmatch m0, phpbb2_posts p) WHERE m0.post_id = p.post_id AND m0.word_id = 16)

BACKTRACE

FILE: includes/db/mysql.php
LINE: 130
CALL: dbal->sql_error()

FILE: includes/search/fulltext_native.php
LINE: 622
CALL: dbal_mysql->sql_query()

FILE: search.php
LINE: 398
CALL: fulltext_native->keyword_search()

When it doesn't find anything it displays correctly that there are no posts matching that criteria.