topsoftbe on "Receive 403 FORBIDDEN after creating new sites"

I have a selfhosted WP and would like to use WP MultiSite. I installed a new wp from scratch, installed MU and then created some new sites. They are created without errors, I see them in "My Sites" I can edit them but when I click on "visit site" I got 403 FORBIDDEN. When I click on the dasboard link for a new site, I got 404 NOT FOUND.
I am using subdomains (mysite.com and then something.mysite.com etc.)
There is one thing I'm not sure if I did it correctly: when I create a new site and look at the info, it gives a slash "/" as path. Is that correct or should it be /something/ ?
I have no idea where to start looking to solve this problem. I am running standard WP for 9 years, never had problems, but with this, I have no idea where to start.
Any help would be greatly appreciated.