Error: HEAD healthcheck failed: This is usually due to network or permission issues

[Richard-Payne]

We have a previously configured Opensearch cluster in Terraform. I'm now trying to make a change to it.
Run terraform plan produces:

Error: HEAD healthcheck failed: This is usually due to network or permission issues. The underlying error isn't accessible, please debug by disabling healthchecks.

Settting TF_LOG_PROVIDER=INFO produces this:

2024-10-08T10:52:34.937+0100 [INFO] provider.terraform-provider-elasticsearch_v2.0.2: 2024/10/08 10:52:34 [INFO] couldn't create client: *errors.withStack, no active connection found: no Elasticsearch node available, *errors.withMessage: timestamp=2024-10-08T10:52:34.937+0100

The provider config is:

provider "elasticsearch" {
  url                     = "https://${aws_opensearch_domain.this.endpoint}"
  aws_profile       = "prod"
  username          = null
  password          = null
  sign_aws_requests = null
  healthcheck       = false
}

I pulled the endpoint attribute out of the tf state record and I can curl it successfully, using https, returning the usual cluster information json blob.

Any help resolving this would be appreicated.