Sorry for the noise: the network problem was already solved in this thread.
So I simply changed caddy.service from this:
Wants=network-online.target systemd-networkd-wait-online.service
to this:
Wants=network-online.target networkmanager-wait-online.service
Now the service starts correctly but the file cannot be executed. This is the topic of this thread.
Sorry, logs are in italian but I hope you can get it:
-- La fase di avvio è done.
apr 17 19:29:15 fedora audit[1]: SERVICE_START pid=1 uid=0 auid=4294967295 ses=4294967295 msg='unit=caddy comm="systemd" exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=? res=success'
apr 17 19:29:15 fedora systemd[4515]: caddy.service: Failed at step EXEC spawning /usr/local/bin/caddy: Resource temporarily unavailable
-- Subject: Il processo /usr/local/bin/caddy non può essere eseguito
-- Defined-By: systemd