凯迪认为一切都很好：

{"level":"info","ts":...,"logger":"tls.issuance.acme.acme_client","msg":"validations succeeded; finalizing order","order":"https://acme-v02.api.letsencrypt.org/acme/order/.../..."}
{"level":"info","ts":...,"logger":"tls.issuance.acme.acme_client","msg":"successfully downloaded available certificate chains","count":2,"first_url":"https://acme-v02.api.letsencrypt.org/acme/cert/..."}
{"level":"info","ts":...,"logger":"tls.obtain","msg":"certificate obtained successfully","identifier":"service.internal.example.com"}
{"level":"info","ts":...,"logger":"tls.obtain","msg":"releasing lock","identifier":"service.internal.example.com"}

在Caddy容器内：

# curl https://localhost/ -H "Host: service.internal.example.com" -v
*   Trying 127.0.0.1:443...
* Connected to localhost (127.0.0.1) port 443 (#0)
* ALPN: offers h2
* ALPN: offers http/1.1
*  CAfile: /etc/ssl/certs/ca-certificates.crt
*  CApath: none
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
* TLSv1.3 (IN), TLS alert, internal error (592):
* error:14094438:SSL routines:ssl3_read_bytes:tlsv1 alert internal error
* Closing connection 0
curl: (35) error:14094438:SSL routines:ssl3_read_bytes:tlsv1 alert internal error

Caddy档案：

{
  acme_dns cloudflare ... # github.com/caddy-dns/cloudflare
}

service.internal.example.com {
  reverse_proxy localhost:1234 # curl localhost:1234 works
}

Wget抛出相同的错误文本，但错误号略有不同：

# wget https://localhost/ --header "Host: service.internal.example.com" -v
--...--  https://localhost/
Resolving localhost (localhost)... 127.0.0.1, ::1
Connecting to localhost (localhost)|127.0.0.1|:443... connected.
OpenSSL: error:14094438:SSL routines:ssl3_read_bytes:tlsv1 alert internal error
Unable to establish SSL connection.

感谢@Buffoonism，我尝试过：

$ curl https://service.internal.example.com --resolve service.internal.example.com:443:127.0.0.1

这个调用（在Caddy容器中）有一个成功的200响应。我已经回退到服务器，结果是一样的：

$ curl https://service.internal.example.com --resolve service.internal.example.com:443:10.1.2.3.4 # works

所以，这似乎是我的线索：主机头service.internal.example.com失败，但curl的resolve标志有效。
我不知道下一步该怎么做