I checked out phpBB topics table, and found the problem. Where in punBB (1.1.5) the topic poster information is stored in text format (the topic starter's name, e.g MisterX), phpBB uses the user's ID-number to do the same.
What causes the trouble is that after the conversion all the topic_poster values were
-1. After I changed one of the topics I started back to my value, the topic appeared to the forum view. So there is a way to correct them. But for crying out loud, do I have to manually change the topic_poster values back to the correct ones? Doing a script to do that is beoynd my knowledge in PHP.
Well, perhaps if I prune the old topics, then I'd have less work to do... 8O
// And I manually set the right values. Now, it works like a dream.