Hi 
this bug has been fixed by @francislavoie and will land in caddy v2.5.1 
See cmd: Fix unix socket addresses for admin API requests by francislavoie · Pull Request #4742 · caddyserver/caddy · GitHub
If you need that fix immediately, you could use xcaddy to build your own caddy binary from the current master branch, which has this fix merged:
xcaddy build master
## or specify the commit sha of the merge
xcaddy build 2e4c0915