Mambu API v2 is not handling a POST request to the /loans/transactions:search endpoint properly

Question

I am working on retrieving an object by sending a request with a filter to Mambu API as is shown in this documentation https://api.mambu.com/?http#mambu-api-v2-loan-transactions. However, it is not responding as it is supposed to. My request is the following (in the example dummy strings replace the originals to avoid any misuse)

curl --location 'https://dummytenant.mambu.com/api/loans/transactions:search' \
--header 'Content-Type: application/json' \
--header 'Authorization: Basic dummyuser:dummypassword' \
--data '{
  "filterCriteria": [
    {
      "field": "encodedKey",
      "operator": "EQUALS",
      "value": "dummyencodedkeyvalue"
    }
  ]
}'

And the response is

{
    "returnCode": 6,
    "returnStatus": "INTERNAL_ERROR"
}

It just sends an INTERNAL_ERROR message. I am not sure if there is something wrong with the request, or maybe the endpoint does not work as I think it should.

Does anyone has worked with this specific API resource, and could solve it?