How to debug a 409 from the TRE CLI?

I'm trying to automate creating some shared services etc with the CLI, and getting a 409 return code with no error message. How can I debug this and find out what the problem is?

Steps to reproduce

My specific example is a certificate service for Nexus. I have a JSON file, as follows:

{
  "templateName": "tre-shared-service-certs",
  "properties": {
    "display_name": "nexus-ssl certificate service",
    "description": "automated deployment",
    "cert_name": "nexus-ssl",
    "domain_prefix": "nexus"
  }
}

I run the CLI:

tre shared-services new --definition-file my-definition.json

and I get this response:

Creating shared_service...
{
  "status_code": 409,
  "body": ""
}

what can I do to debug this?

This is using the latest version of the CLI, installed just a few minutes ago.

N.B. Other commands work fine, creating workspaces etc, so my basic installation is good and I'm logged in to the TRE.

Start by reproducing the empty 409 with tre shared-services new --definition-file my-definition.json and the supplied certificate-service definition. Trace how the CLI handles the shared-service creation response; done means the failure exposes an actionable error or a clear way to debug the cause.