The subdomain redirects to the main site, Bitrix. how to fix?

Question

1C-Bitrix

A subdomain is configured on the server. Links to the main Bitrix folders. If we remove the Bitrix prologue in the php index, everything works. If not, Bitrix redirects to the main site. Created two sites in the system. 1 site - main
ID: s1
Active: yes
Name:
Parameters for defining a site in the public section
Default:
Domain name:
(list of domain names, each in a new line) site.ru
Site folder: /
Sorting: 1
Path to the web root folder -servers for this site:
(leave blank if all sites have the same root folder) /home/bitrix/www/
Parameters
Website name:
server URL (without http://): site.ru
E-Mail address by default:
2 site - subdomain
ID: op
Active: yes
Name:
Parameters for defining a site in the public section
Default:
Domain name:
(list of domain names, each in a new line) opt.site.ru
Site folder: /
Sort: 2
Path to the web server root folder for this site:
(leave empty if all sites have the same root folder) /home/bitrix/ext_www/opt.site.ru/
Parameters
Website name:
server URL (without http://): opt.site.ru Default
E-Mail address:
Can someone tell me which way to look?
Is this possible due to the use of Let's Encrypt ssl certificate?

Answer 1

Perhaps it will come in handy for someone:
check /bitrix/admin/security_hosts.php (Settings - Proactive Defense - Hosts / domains), if there is a "limit" checkbox and "redirect" is checked, it will redirect to the specified domain. Either disable the restriction, or add your subdomain to the allowed list.

Answer 2

show .htaccess