This was the first time. I have just tried a second time, and now it works
correctly:
IMPORTANT NOTES:
- Congratulations! Your certificate and chain have been saved at:
/etc/letsencrypt/live/s20.bistruphave.dk/fullchain.pem
Your key file has been saved at:
/etc/letsencrypt/live/s20.bistruphave.dk/privkey.pem
Your certificate will expire on 2021-06-06. To obtain a new or
tweaked version of this certificate in the future, simply run
certbot again. To non-interactively renew all of your
certificates, run "certbot renew"