Try a config like this
# /etc/httpd/conf.d/monitoring.domain.com
<VirtualHost *:80>
ServerAdmin monitoring-admin@domain.com
DocumentRoot /var/www/html
ServerName monitoring.domain.com
Include /omd/apache/*.conf
RewriteEngine On
RewriteCond %{SERVER_PORT} !^443$
RewriteRule (.*) https://%{HTTP_HOST}/$1 [L]
RequestHeader set X-Forwarded-Proto "https"
</VirtualHost>
<VirtualHost *:443>
ServerAdmin monitoring-admin@domain.com
DocumentRoot /var/www/html
ServerName monitoring.domain.com
SSLEngine on
SSLCertificateFile /etc/ssl/monitoring.domain.com.crt
SSLCertificateChainFile /etc/ssl/monitoring.domain.com.chain
SSLCertificateKeyFile /etc/ssl/monitoring.domain.com.key
Include /omd/apache/*.conf
</VirtualHost>
SSLProtocol all -SSLv3 -TLSv1 -TLSv1.1
SSLCipherSuite ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384
SSLHonorCipherOrder off
maybe you have to remove the welcome.conf