Thanks a lot Chuck, I have made that modification, guess this solves the problem.
Sorry for my confusing description. I'll try again:
This situation occurs when a user is logged out of the forum and, for some unknown reasons, the "member_id" cookie is missing. All other IPB cookies are present.
Classifieds, again for unknown reasons, think that the user is logged in and let him post an ad. But since the "member_id" cookie is missing, the user name is missing from the "showproduct" display, field is empty.
But in the main listing it says that the last ad is posted by "Unregistered".
All these ads that I have been able to trace have been posted by members that has been logged out when visiting the Classified section. None of them have been made by pure guests.
(I said in post 11 that the user in question was logged in, but further investigation showed that this was wrong, he was not logged in.)