Dirk,
Hi. More than likely this is because your DotNetNuke installation includes an object qualifier. Object qualifiers add a prefix to every single stored procedure, table, and so forth within your dnn installation. You can see if your site uses an object qualifier either by checking in your web.config or go to Host, SQL and do a query such as this:
select top 10 * from sysobjects where name like '%portals%'.
If you get results such as a table name called 'portals' then your system isn't using object qualifiers but if you get something such as dnn_portals then your object qualifier is 'dnn_' and you must use that in other queries throughout the system. For example the query to return regions/countries would not pull from the 'lists' table, it would pull from the 'dnn_lists' table. Hope this helps, if you still have a problem please submit a support ticket and we can review it in more detail.
-Chad