Sometimes it is the last change that makes the difference - https is now also working within the local network (Proxy-settings and Mattermost server on the same system)
<VirtualHost *:443>
# If you're not using a subdomain you may need to set a ServerAlias to:
# ServerAlias www.mydomain.com
ServerName vmmm-rd1.privat-net.intranet
ServerAdmin webmaster@localhost
# DocumentRoot /var/www/vmmm-rd1.privat-net.intranet-ssl
# SSL Section
SSLEngine on
SSLCertificateFile /etc/ssl/certs/vmmm-rd1.privat-net.intranet-self.crt
SSLCertificateKeyFile /etc/ssl/private/vmmm-rd1.privat-net.intranet-self.key
# Set web sockets
RewriteEngine On
RewriteCond %{REQUEST_URI} /api/v[0-9]+/(users/)?websocket [NC]
RewriteCond %{HTTP:UPGRADE} ^WebSocket$ [NC,OR]
RewriteCond %{HTTP:CONNECTION} ^Upgrade$ [NC]
RewriteRule .* ws://127.0.0.1:8065%{REQUEST_URI} [P,QSA,L]
<Location />
Require all granted
ProxyPass http://127.0.0.1:8065/
ProxyPassReverse http://127.0.0.1:8065/
ProxyPassReverseCookieDomain 127.0.0.1 vmmm-rd1.privat-net.intranet
</Location>
</VirtualHost>
I used the config file already in the past - but it looks like that the changes in the mattermost config fixed it now.
I will check tomorrow if I can get is working as well from the internet.