I installed multisite WP on my localhost, main site

I installed multisite WP on my localhost, main site is on http://wpmutest and I created
subdomain site at http://portal.wpmutest (localhost domains).

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?

EDIT:

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.