Workaround for certificate renewal error
I've just spent too many minutes of my life dealing with this ultimately trivial issue, so let me record it for 89 days from now when it will come back.
I got a friendly email from the Let's Encrypt Expiry Bot telling me that the certificates on this webpage are about to expire. Since I had used certbot to get the certificates issued in the first place, the natural attempt was
sudo certbot renew
which failed miserably with
Failed to renew certificate aghitza.org with error: Problem binding to port 80: Could not bind to IPv4 or IPv6.
After alternately googling the error and making random changes to my
lighttpd.conf, I eventually realised that I should stop the web server, run the renewal command, then start the web server again:
sudo systemctl stop lighttpd sudo certbot renew sudo systemctl start lighttpd
Now I know.