I found out why this is happening:
This SQL-QUERY:
Query INSERT INTO phpbb_sessions (session_user_id, session_start, session_last_visit, session_time, session_browser, session_forwarded_for, session_ip, session_autologin, session_admin, session_viewonline, session_page, session_forum_id, session_id) VALUES (1, 1353938641, 1353938641, 1353938641, 'Mozilla/5.0 (<My_OS>) Gecko/20100101 Firefox/15.0', '', '<My_IP>', 0, 0, 1, '../4images/index.php', 0, 'bdd93ee4a4525d342586452f8f88b8bd')
Unreadable session reason in phpbb_sessions.php
What should I do to fix this?