OK, so I was able to replicate the problem and I understand why it happens now.
In SMF, when you delete an account (and choose not to delete this user's posts), all posts that he created are marked with "id_member" = 0 (in smf_messages table).
Apparently, IPB doesn't
So your account ID=2 is a removed account, his posts were not deleted and IPB's posts table kept his ID associated with his posts. When you converted to SMF, the relevant tables were copied pretty much directly (no verification is performed) so this "detail" was also transferred.
When SMF shows you a topic, it loads also, for each post, its author's details. If the post's author ID is 0, then it's a guest and only "poster_name" is shown. If the ID is != 0, then it tries to load the user's details. Well, if the user was removed, the details cannot be found
So, this is your issue, apparently: your "smf_messages" table has posts recorded from users that no longer exist.
This can automagically be fixed by SMF if you run the Maintenance Task "Find and repair any errors". For example, in my test forum where I deliberately caused this error:
The following errors are fouling up your forum:
Message #11 was posted by member #4, who is now missing.
Of course, a good database backup should be performed first. Just in case
