Once I create a network I cannot admin plugins anymore

Hi, I have described the problem here, but reposting because that topic is marked ‘resolved’ and it isn’t for me.
http://wordpress.org/support/topic/cant-add-plugins

I have downloaded WP 3.1.3 and installed it manually. All good, the plugin admin panel is as expected. There is only one user in the database, the admin user. Then I create a network using subfolders, change wp-config and htaccess and login again. It all seems to work, but now I cannot admin the plugins anymore – the plugin entry in the menu doesn’t have the drop down submenus anymore

thanks.