Hi Janusz,
thanks for your response.
1. I see, works indeed as you explained
2. I found out that there were indeed 4 item categories (Components, Charges, Systems, Services). They did show up in the category combo-box on the item page, but they DID NOT SHOW UP on the item category page (were not inactivated), so something appears to be wrong. Maybe related to this is that earlier I removed the tax type "Regular". Those 4 item categories depend on tax type "Regular" but nevertheless tax type "Regular" was removed, instead of giving a message that it cannot be removed because of a dependency.
My problem has been solved however by manually removing all rows from the stock_category table. After doing so I could delete all remaining original accounts.
Hendrik