Now that I am getting more familiar with IP.Forum's converter, I can see what happened.
The problem was with one specific thread in our vBulletin database, which had thread.attach set to 65535. Upon further inspection, I see that it actually doesn't have any attachment at all, so I manually set thread.attach to 0 on this thread.
Therefore, the problem was not with IP.Board converter, but with our database, which had a bug.
I created a simple script to fix all my rows that I, by mistake, had set to '1' instead of the number present in thread.attach.
So, this was obviously "my" mistake, so you can close this case, as it is not a bug with the converter.