Hi,
The error is most likely happening because you are running mojoPortal in a sub folder instead of as a root site and the root site above it also has a Web.config file which conflicts.
You can safely comment out the machine key in most cases. However if you are going to use encrypted passwords its best if the machine key is there because it is used in encrypting things and if it changes you will not be able to decrypt.
If you comment this out you still may encounter other conflicts due to the Web.config file living in the folder c:\webs\talkactive\wno114835\hjernebarna.no, so its best if you can remove that file or move mojoPortal up to that folder.
Hope it helps,
Joe