HMMMM.. it has to be something wrong with the msSQL_server_setup.asp file.
I tried again, restoring data to the "forum2" database, by using the backup from "forum1", after that I ran the msSQL_server_setup.asp modified to only update the section of the Stored Procedures.
I get the same error, but also noticed I got a new error this time:
Error Creating the Stored Procedure wwfSpDBinfo (it may already exsist). If you are using MS SQL Server 7 ignore this error (You will not be able to view SQL Server stats in the admin area)
The result is the same though.
Now.. It seems like there is either something missing in the stored procedures/tabel, or the file forum_topic.asp is corrupted somehow.
I really need a good advice now, as I have no more ideas what to do from here.