I had imported a phpbb forum to mybb previously. The importer script
messed up the firstpost ids. In this case most topics cannot be imported
to discourse as the script is not able to determine the topic post.
In case you are infected of this problem just uncomment this mysql
query. It will retrieve the first_post_id manually but it will slow down
the import.
The post_ids in my mybb installation did not be in the right order. Post
with ID 2 was posted after post 5. I think it has something to do with a
previous import from phpbb to mybb.
