I can access main site & wp-admin, and portal.wpmutest subdomain site also works - on the frontend.
But when I try to access wp-admin of portal subdomain website, it just reloads wp-login page.
I tried superadmin account and later I alaso assigned new user to that site as Administrator.
None of them can login - the login page just gets reloaded.
What are possible causes of this?
the website recognizes valid login details vs invalid. With invalid it shakes the form & says 'incorrect' details, but when I enter correct details it just reloads empty form.