I have already reported this as a problem with another website.
I have a multisite installation on http://pontypridd.ourlocal.town/
It all works fine until I install New Blog Templates. If I then sign up as a new user (http://pontypridd.ourlocal.town/wp-signup.php) and create a new website, I cannot access the admin area as that new user.
Logged in as super admin all is fine, but as my newly created user, it does not work.