Matrix synapse federation not possible

Question

I successfully set up my homeserver based on the description available in https://linuxhandbook.com/install-matrix-synapse-docker/ using a docker image.

I had to change it a little bit, as I am using Apache as a reverse proxy. This I made from another example:

<VirtualHost *:443> 
    SSLEngine on 
    ServerName matrix.oe8bck.at
Include /etc/letsencrypt/options-ssl-apache.conf
SSLCertificateFile /etc/letsencrypt/live/...
SSLCertificateKeyFile /etc/letsencrypt/live/...

RequestHeader set &quot;X-Forwarded-Proto&quot; expr=%{REQUEST_SCHEME}
AllowEncodedSlashes NoDecode
ProxyPreserveHost on
ProxyPass /_matrix http://127.0.0.1:8008/_matrix nocanon
ProxyPassReverse /_matrix http://127.0.0.1:8008/_matrix
ProxyPass /_synapse/client http://127.0.0.1:8008/_synapse/client nocanon
ProxyPassReverse /_synapse/client http://127.0.0.1:8008/_synapse/client

&lt;IfModule security2_module&gt;
  SecRuleEngine off
&lt;/IfModule&gt;


</VirtualHost>
<VirtualHost *:8448>
    SSLEngine on
    ServerName oe8bck.at
Include /etc/letsencrypt/options-ssl-apache.conf
SSLCertificateFile /etc/letsencrypt/live/...
SSLCertificateKeyFile /etc/letsencrypt/live/...

RequestHeader set &quot;X-Forwarded-Proto&quot;expr=%{REQUEST_SCHEME}
AllowEncodedSlashes NoDecode
ProxyPass /_matrix http://127.0.0.1:8008/_matrix nocanon
ProxyPassReverse /_matrix http://127.0.0.1:8008/_matrix

&lt;IfModule security2_module&gt;
  SecRuleEngine off
&lt;/IfModule&gt;


</VirtualHost>

I checked the TLS certificate, it is valid. But the federation does not work! I checked it with https://federationtester.matrix.org/api/report?server_name=matrix.oe8bck.at but I cannot see the reason for these errors:

"Message": "Get \"https://88.116.94.22:8448/_matrix/key/v2/server\": dial tcp 
88.116.94.22:8448: connect: connection refused"
  "error": "Get \"matrix://matrix.oe8bck.at/_matrix/federation/v1/version\": dial tcp 88.116.94.22:8448: connect: connection refused"

Maybe somebody can help me?