I got rid of the error message on Ubuntu 20.04 by removing the ppa repository. It took me a long time to find out the exact command argument, so this might help others :-).
I used the command:
sudo apt-add-repository -r ppa:certbot/certbot
After that, the following commands do not generate any errors:
sudo apt update
sudo apt-get update
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…