In that case you're probably running a very old version of Apache (less than 2.4.8). If this is the case, you need additional configuration (SSLCertificateChainFile):
For your setup:
Change:
SSLCertificateFile /etc/letsencrypt/live/www.dimsum.dk/fullchain.pem
SSLCertificateKeyFile /etc/letsencrypt/live/www.dimsum.dk/privkey.pem
to
SSLCertificateFile /etc/letsencrypt/live/www.dimsum.dk/cert.pem
SSLCertificateChainFile /etc/letsencrypt/live/www.dimsum.dk/chain.pem
SSLCertificateKeyFile /etc/letsencrypt/live/www.dimsum.dk/privkey.pem
Edit: Wait, you're saying you run Ubuntu 20.04? The Apache on that Ubuntu shouldn't be that old?