Vault database connection resource returns connection error 9999

Terraform
1

I have this vault database connection resource:

resource "vault_database_secret_backend_connection" "dbc" {
  backend       = vault_mount.db.path
  name          = var.db_name
  allowed_roles = [ "writer", "reader" ]
  verify_connection = true

  mysql {
    # connection_url = "{{username}}%40${var.db_name}:{{password}}@tcp(${azurerm_mysql_server.spinn-db.fqdn}:3306)/"
    connection_url = "{{username}}:{{password}}@tcp(${azurerm_mysql_server.spinn-db.fqdn}:3306)/"
  }
  root_rotation_statements = [
    "SET PASSWORD FOR '{{username}}' = '{{password}}';"
  ]
  data = {
    username = var.login
    password = var.pass
  }
}

Running terraform returns this error:

2020-01-08T08:15:04.691-0500 [DEBUG] plugin.terraform-provider-vault_v2.7.0_x4: 2020/01/08 08:15:04 [DEBUG] Vault API Response Details:
2020-01-08T08:15:04.691-0500 [DEBUG] plugin.terraform-provider-vault_v2.7.0_x4: —[ RESPONSE ]--------------------------------------
2020-01-08T08:15:04.691-0500 [DEBUG] plugin.terraform-provider-vault_v2.7.0_x4: HTTP/2.0 400 Bad Request
2020-01-08T08:15:04.691-0500 [DEBUG] plugin.terraform-provider-vault_v2.7.0_x4: Content-Length: 161
2020-01-08T08:15:04.691-0500 [DEBUG] plugin.terraform-provider-vault_v2.7.0_x4: Cache-Control: no-store
2020-01-08T08:15:04.691-0500 [DEBUG] plugin.terraform-provider-vault_v2.7.0_x4: Content-Type: application/json
2020-01-08T08:15:04.691-0500 [DEBUG] plugin.terraform-provider-vault_v2.7.0_x4: Date: Wed, 08 Jan 2020 13:15:04 GMT
2020-01-08T08:15:04.691-0500 [DEBUG] plugin.terraform-provider-vault_v2.7.0_x4: Server: openresty/1.15.8.2
2020-01-08T08:15:04.691-0500 [DEBUG] plugin.terraform-provider-vault_v2.7.0_x4: Strict-Transport-Security: max-age=15724800; includeSubDomains
2020-01-08T08:15:04.691-0500 [DEBUG] plugin.terraform-provider-vault_v2.7.0_x4:
2020-01-08T08:15:04.691-0500 [DEBUG] plugin.terraform-provider-vault_v2.7.0_x4: {
2020-01-08T08:15:04.691-0500 [DEBUG] plugin.terraform-provider-vault_v2.7.0_x4: “errors”: [
2020-01-08T08:15:04.691-0500 [DEBUG] plugin.terraform-provider-vault_v2.7.0_x4: “error creating database object: error verifying connection: Error 9999: An internal error has occurred. Please retry or report your issues.\u0000”
2020-01-08T08:15:04.691-0500 [DEBUG] plugin.terraform-provider-vault_v2.7.0_x4: ]

I know the database is up and accepting connections. What does the error mean or how can I troubleshoot it?