BTW, the reason I can’t use the working renewal method anymore (where some old letsencrypt version is buried inside some Docker container) is because it’s now complaining about ACME 1 which they say will become obsolete next month.
On the Ubuntu 16.04, there is certbot 0.31.0 and I hope I can use this directly now to renew.
If you can, can you try to upgrade the server OS? (like to 18.04 LTS or the latest 20.04 LTS)
Are you sure you are used certbot before?
If so, according to your setup, the command might be similar to this: certbot --config-dir /home/david/letsencrypt --server https://acme-v02.api.letsencrypt.org/directory
Use this instead of regular certbot
A brief explaination: --config-dir Configuration directory. (default: /etc/letsencrypt) --server ACME Directory Resource URI. (Force certbot to use ACMEv02)