Please check your vB database, table "editlog".
Looking at the code:
---* {$to_prefix}messages (update id_msg)
SELECT postid AS id_msg, username AS modified_name, dateline AS modified_time
FROM {$from_prefix}editlog
ORDER BY dateline;
---*
It seems that you have a NULL value in column "dateline", maybe?
Can you please remove that code momentarily and try the converter again? Just to be sure...