Message: log in and try again.

I am logged in! What is wrong?
All prereq checked in ok.
Change the name of the site and typed a slogan.
Klicked save and got the mentioned message.
????

I reinstalled the site. Preq. test went well even regarding the db.
This as described. Screenshot of the error attached. The login page loads in a separate window.
Rebuild db. How in more detail?
Reload admin? How in more detail?
I ask myself why does this not work the first time?

Rebuild db. How in more detail?
Reload admin? How in more detail?

Please read the first tutorial to get to grips with these. Rebuilding the database happens by visiting the url - http://yoursite.com/dev/build. A common fix for CMS errors is to reload the admin but with a flush=all by visiting the url - http://yoursite.com/admin?flush=all. Then logging in.

The cms login does not work unless I check the "Husk meg neste gang".

Could be a bug with some translations somewhere. If you can't get it to work after following the above 2 steps then try install in english first and see if the problem still exists.

This as described. Screenshot of the error attached. The login page loads in a separate window.

That error message is saying you're not logged in. So something is logging you out, perhaps your browser has cookies disabled or session support on your server is turned off?

I can't switch to english version - I then will be forced to do the translation myself.

I meant install it using the en version. You can always change language later on. If it works in en but not that language you're using that would help the core developers track down your bug.