Site's wp-admin keeps redirecting to admin's wp-admin URL

Hi guys,

I run WP multisite like this:
admin(main) page: https://www.biegunjiri.cz
site: https://www.bieguninvest.cz (test1.biegunjiri.cz)

Everything works fine but when I go to site admin bieguninvest.cz/wp-admin , I get redirected to test1.biegunjiri.cz (and after login there is still test1 and admin’s url).

I really need this to keep separate for each site (and for 2 other sites I will do soon).
Do you guys have any advice for this?

My htaccess:
# BEGIN WordPress
Options +FollowSymLinks
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ – [L]
# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ – [L]
RewriteRule ^[_0-9a-zA-Z-]+/(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^[_0-9a-zA-Z-]+/(.*\.php)$ $1 [L]
RewriteRule . index.php [L]
# END WordPress