Hashicorp Vault won't let me delete a Policy even using the root token

Question

I am trying to delete a policy.

After logging in with the root token, I do the following:

$ vault policy delete testttt
Error deleting testttt: Error making API request.

URL: DELETE https://vault.local:8200/v1/sys/policies/acl/testttt
Code: 400. Errors:

* failed to delete policy: AccessDenied: Access Denied
    status code: 403, request id: VB6YWECETDJ5KB7Q, host id:         
S0FJvs41pSbzTmP1lDr/aVSOPjeRVz4Vk/ofkFHu8jvNjfzk6ARnY33qzP/usqmpVDExwLlsF44=

My config file looks like this:

storage "s3" {
  access_key = "XXXX"
  secret_key = "XXXX"
  bucket     = "XXXX-vault"
  region     = "eu-central-1"
}

listener "tcp" {
  address     = "0.0.0.0:8200"
  tls_cert_file = "/etc/vault.d/fullchain.pem"
  tls_key_file  = "/etc/vault.d/privkey.pem"
}


api_addr = "http://0.0.0.0:8200"
cluster_addr = "https://0.0.0.0:8201"
ui = true

Something seems totally off, as after using the root token in the UI, I also just see this:

null is not an object (evaluating 'l.userRootNamespace')