Reactivate apache2 after letsencrypt installation

try setting;

SSLCertificateFile /etc/letsencrypt/live/xy.no-ip.biz/cert.pem
SSLCertificateKeyFile /etc/letsencrypt/live/xy.no-ip.biz/privkey.pem
SSLCACertificateFile /etc/letsencrypt/live/xy.no-ip.biz/chain.pem