caddy.service start request repeated too quickly

For anyone else pulling their hair out, note the lines StartLimitInterval and StartLimitBurst in the caddy.service file - if you're testing and repeatedly stop / starting you will hit the StartLimitBurst limit. Need to increase it to lots, or change the StartLimitInterval, to allow this.

In my /etc/systemd/system/caddy.service file, I had the following line:

Restart=on-failure

Commenting that out (with # or ;) and restarting the service showed the underlying problem, which was in my Caddyfile.

EDIT:

service caddy status only prints a few lines from the log, so sometimes you can find the underlying problem by simply looking at the full log. If using syslog, this is done with:

journalctl -u caddy