Cause
In recent Apache version, Apache team has released fixes for CVEs that affected Apache + nginx functionality: new changes do not allow Apache process requests from nginx without the server name (by default, nginx does not pass the server name through SNI when establishing a connection with a proxied HTTPS server).
Plesk team is preparing a custom fix for Plesk Obsidian 18.0.70 and 18.0.71.
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Resolution
Add proxy_ssl_server_name, proxy_ssl_name and proxy_ssl_session_reuse directives in nginx configuration to make nginx pass the server name to Apache through TLS Server Name Indication (SNI) extension:
Connect to the server via SSH
Run the script (without any modifications):
echo -e "proxy_ssl_server_name on;nproxy_ssl_name $host;nproxy_ssl_session_reuse off;" > /etc/nginx/conf.d/fixssl.conf && systemctl restart nginx