400 from email template API (from terraform)

Does the terraform provider only surface the error response status code or do you mean that it failed and the response did NOT include a body with any additional details. If there’s no response body or the body is not surfaced by the tooling this can be a bit more complex to troubleshoot.

Is it possible for you to share the tenant name where this happens? This could allow us to check service logs; if you’re wary of sharing a production tenant name you can always create a temporary tenant just to reproduce this situation and share that one.