Thread authors displayed as guest after Reparse BBCode

After using Reparse BBCode in STK thread authors are displayed as guest in viewforum.php. However, in viewtopic.php they are displayed correctly.

When looking into my mySQL database it shows that at least some of the info in the topics table is still intact.

mysql> select topic_poster from phpbb_topics;
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...

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The posts table does not seem to contain "deleted user" oder "anonymous user".

mysql> select poster_id from phpbb_posts where poster_id='0';
Empty set (0.00 sec)

mysql> select poster_id from phpbb_posts where poster_id='-1';
Empty set, 1 warning (0.00 sec)

What can I do to get the topic author displayed correctly again in viewforum?