I'm having problems with an MSM install into an existing site (EE 2.9 / MSM 2.1.7) where non super admins can't switch to the added sites.
The links in the site switching dropdown look like
admin.php?/cp/sites&site_id=2&page=XXX and when you click on them as a super admin you get redirected to whatever page you're on. But when logged in as a non-super admin you actually end up on
admin.php?/cp/sites&site_id=2&page=XXX which is where the error message appears.
cp_session_type to cookies only and I've checked that the member group has the necessary privileges.
What else should I be looking at?