systemctl status … will display an excerpt.
If the error is displayed when starting Caddyserver from a console, but is not shown in the logs if an initdaemon such as systemd is used (which logs stdout and stderr), then Caddyserver’s authors have forgotten to Flush stdout and/or stderr before exiting.
found: https://caddy.community/t/cant-start-caddy-with-systemd-service-file/3447
I wish I’d found this little gem a long time ago!