Network site is gone

Hi all
I have just change a site into a multisite. The main site is working just fine. So is all of the admin area.

But I can’t go to the login page ( https://musikerblogs.dk/nyblog/wp-admin/ ) and the network site looks as if there is no theme installed ( https://musikerblogs.dk/nyblog/ ).

Here are some possible sources of the error:
I changed the original setup from subdomain to subdirectory by changing to ‘false’ in wp-config.php.
For some reason all pages (not only posts) has added /blog/ in the url. I can’t leave it out in the Permanent links settings.
… or maybe I’ve just miss-set some settings

Nevertheless, I’m really lost!
Where to look?