We’re using ReverseProxyPlug in a Phoenix server, and we have been configuring it like so:

opts =
  ReverseProxyPlug.init(
    upstream: url,
    allowed_origins: allowed_origins,
    proxy_url: "#{cors_scheme}://#{cors_host}:#{cors_port}",
    # We need to force the host
    # used for ssl verification here so that the connection isn't rejected.
    # Note that we have to convert the authority to a charlist, since this uses Erlang's `ssl` module
    # internally, which expects a charlist.
    client_options: [
      ssl: [
        {:server_name_indication, to_charlist(authority)},
        {:versions, [:"tlsv1.2", :"tlsv1.3"]}
      ]
    ]
  )

Thus, we’ve been getting the log message

~c"Server authenticity is not verified since certificate path validation is not enabled"
Reason: ~c"The option {verify, verify_peer} and one of the options ‘cacertfile’ or ‘cacerts’ are required to enable this."

So, I’ve updated to

opts =
  ReverseProxyPlug.init(
    upstream: url,
    allowed_origins: allowed_origins,
    proxy_url: "#{cors_scheme}://#{cors_host}:#{cors_port}",
    # We need to force the host
    # used for ssl verification here so that the connection isn't rejected.
    # Note that we have to convert the authority to a charlist, since this uses Erlang's `ssl` module
    # internally, which expects a charlist.
    client_options: [
      ssl: [
        {:server_name_indication, to_charlist(authority)},
        {:versions, [:"tlsv1.2", :"tlsv1.3"]},
        {:verify, :verify_peer},
        {:partial_chain, :auto},
        {:cacerts, :public_key.cacerts_get()}
      ]
    ]
  )

I’ve ensured that root certificates are installed on the system, and are used by openssl:

$ openssl s_client -connect www.archive.org:443

…

Verify return code: 0 (ok)

But now the proxy request fails, sending a status 502, and we’re still getting the log message

~c"Server authenticity is not verified since certificate path validation is not enabled"
Reason: ~c"The option {verify, verify_peer} and one of the options ‘cacertfile’ or ‘cacerts’ are required to enable this."

What should I look at to debug this?