FYI If you are following the official documentation to setup your own #mastodon #instance. Be sure to check out this GitHub issue when you get to the "Setting up nginx" step.

This command Does Not Work
certbot --nginx -d example.com

Do this instead: (as root)
systemctl stop nginx
certbot certonly --standalone -d example.com
nano /etc/nginx/sites-available/mastodon

Then manually uncomment the two ssl lines
systemctl start nginx

https://github.com/mastodon/mastodon/discussions/17372#discussioncomment-2045474