Listen 80
Listen 443

<virtualhost *:80>
  ProxyPreserveHost On
  ServerName sub.example.de
  Redirect permanent / https://sub.example.de/
</virtualhost>

<IfModule mod_ssl.c>
<VirtualHost *:443>
    ServerName sub.example.de
    ProxyPreserveHost On
    SSLEngine On

    ProxyPass / http://localhost:8888/
    ProxyPassReverse / http://localhost:8888/

    SSLCertificateFile /etc/letsencrypt/live/sub.example.de/fullchain.pem
    SSLCertificateKeyFile /etc/letsencrypt/live/sub.example.de/privkey.pem
</VirtualHost>
</IfModule>