When you have enabled strict SQL checking on the mysql database (ONLY_FULL_GROUP_BY) then you get an db error when you hit the "own posts" button in the forum:

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SQL ERROR [ mysql4 ]

'phpbb3_db.p.post_id' isn't in GROUP BY [1055]

SQL

SELECT DISTINCT SQL_CALC_FOUND_ROWS p.post_id FROM phpbb_topics t, phpbb_posts p WHERE p.poster_id = '2' AND t.topic_id = p.topic_id GROUP BY t.topic_id, t.topic_last_post_time ORDER BY t.topic_last_post_time DESC

BACKTRACE

FILE: [ROOT]/includes/db/mysql.php
LINE: 175
CALL: dbal->sql_error()

FILE: [ROOT]/includes/search/fulltext_native.php
LINE: 1009
CALL: dbal_mysql->sql_query()

FILE: [ROOT]/search.php
LINE: 513
CALL: fulltext_native->author_search()
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When you look at the sql statement you can see that it's not valid sql
compared to sql standards.

Best regards
McA