Can't federate server

Hello,

the following problem:

my server can not be reached for federation. The log of a friend says “No server to federate with found” if he tries to add my server.
My configuration is the following:
virtual box ubuntu 18 with apache and nextcloud
windows server 2016 with apache as reverse proxy for https

thats my apache config on windows:

<VirtualHost *:80>
	ServerName "***"
	ServerAlias "***"
	
	DocumentRoot ***

<Directory ***> 
    Options All 
    DirectoryIndex index.php index.html
    AllowOverride All 
    Require all granted 
</Directory> 

RedirectMatch 301 ^/(?!\.well-known\/acme-challenge)(.*) https://***/$1	
<VirtualHost *:443>
	ServerName ***
	ServerAlias "***"

#hsts
Header always set Strict-Transport-Security "max-age=31536000; includeSubDomains"

DocumentRoot "***"
<Directory "***">
		Options Indexes FollowSymLinks MultiViews
		AllowOverride All
		Require all granted
</Directory>

ProxyPreserveHost On
ProxyPass / http://192.168.1.12/
ProxyPassReverse / http://192.168.1.12/

SSLEngine On
SSLCertificateFile "***-crt.pem"
SSLCertificateKeyFile "***-key.pem"
SSLCertificateChainFile "***-crt.der"

and my config.php for nextcloud:

<?php
$CONFIG = array (
  'instanceid' => '***',
  'passwordsalt' => '***',
  'secret' => '***',
  'trusted_proxies' => 
  array (
    0 => '192.168.1.13',
  ),
  'trusted_domains' => 
  array (
    0 => 'cloud.***',
  ),
  'overwriteprotocol' => 'https',
  'overwritehost' => 'cloud.***',
  'overwritecondaddr' => '^192.168.1.13$',
  'memcache.local' => '\\OC\\Memcache\\APCu',
  'datadirectory' => '/media/sf_cloud',
  'overwrite.cli.url' => 'http://192.168.1.12',
  'dbtype' => 'mysql',
  'version' => '13.0.2.1',
  'dbname' => 'cloud',
  'dbhost' => '192.168.1.13',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'mysql.utf8mb4' => true,
  'dbuser' => '***',
  'dbpassword' => '***',
  'installed' => true,
  'maintenance' => false,
  'mail_domain' => 'cloud.***',
  'mail_smtpmode' => 'sendmail',
  'mail_smtpauthtype' => 'LOGIN',
  'mail_from_address' => 'noreply',
  'mail_smtpsecure' => 'ssl',
  'updater.secret' => '***',
  'loglevel' => 1,
  'theme' => '',
);

has anybody some ideas what’s wrong?

Kind regards, Steve.

Does nobody have an idea?