To make it short, I can log in to my separate sites, but not anymore to the network admin.
When I try to log in the network admin, it says: "You do not have sufficient permissions to access this page."
Strangely enough, I didn't made any change of any sort, not even installing a plugin. It worked before.
Also: I used a username before, but now my username has been changed to admin while I never used it and that it is forbidden as a username.