Using web site as reverse Proxy, get server error 500

I followed https://www.howtoforge.com/community/threads/ispconfig-with-reverseproxy.78182/
My modified Apache Options in the Option Tabs are below, as I not have SSL by now

Code:

#RewriteEngine On
#RewriteCond %{HTTPS} off
#RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}
#SSLProxyEngine On
RequestHeader set Front-End-Https "On"
ProxyPreserveHost On
ProxyRequests Off
ServerName gwmail.muekno.de
#ServerAlias controlpanel.ispcp.domain.name
ProxyPass / https://172.16.1.232/
ProxyPassReverse / https://172.16.1.232/
<Proxy *>
Order deny,allow
Allow from all
</Proxy>

If I open https://172.16.1.232/ (the target) everything works fine
Till now I had a separate server outside the ISPconfig servers that works fine too with this configuration

Code:

<Virtualhost 172.16.1.241:80>
ServerName gw.gerdakloos.de
ServerAdmin [email protected]
ProxyPreserveHost On
DocumentRoot /var/www/html
ProxyPass /.well-known !
ProxyPass / http://gw2.gerdakloos.de/ retry=0 disablereuse=On
ProxyPassReverse / http://gw2.gerdakloos.de/

##RewriteEngine on
##RewriteCond %{SERVER_NAME} =gw.gerdakloos.de
##RewriteRule ^ https://%{SERVER_NAME}%{REQUEST_URI} [END,NE,R=permanent]
RewriteEngine on
RewriteCond %{SERVER_NAME} =gw.gerdakloos.de
RewriteRule ^ https://%{SERVER_NAME}%{REQUEST_URI} [END,NE,R=permanent]
</Virtualhost>

The reason is this server does nothing than proxy and I want to get rid of is, to save RAM on my ESXi, so as there is a webserver in my ISPconfig3 multiple server system, why not user an additional website as proxy.
This configuration has a working letsencrypt certificate too, what ist planed for the new proxy too.
What I do not know, there is nothing in the tutorial , what to do with the default generated index.html
These thread https://www.howtoforge.com/community/threads/use-ispconfig-as-reverse-proxy.78308/ does not help too
running ISPconfig 3.2 on Debian 10 latest patches, mod_proxy and mod_proxy_http are installed and enabled.
Need help hint
Thanks
Rainer

 

Sorry I mixed some IP addresses, correcting this leads to other problems.
But any hint to my configuration is wellcome

 

Have istalled a certificate now and uncomendet the relvant lines. still problems but not ISPconfig relevant
Sorry to disturb you
Happy Christmas and a better new Year than ist was 2020