New 2.8.1 Install, I can log in once and then am forbidden

I am installing a fresh site with wpmu-2.8.1.
The install (with subdirectories, I don't have a choice there) goes well.
I can log in once. This time I did not log out immediately but tried to log in with once of two users, admin and a new one, on another computer.
I get the 403 Forbidden You don't have permission to access /wp-admin/ on this server with both users.
I have repeated this 3 times now.....any ideas?