Get Token to download a file fails if file is associated with a Document

Hi, I am trying to download a file from Aras. To donwload a file we first need a token. I am getting this token from the AuthenticationBroker Service. However, the service is failing to issue the token with an error on Aras server. This occurs only if the file is associated with a Document or CAD. If the file is standalone (created from Admin UI - FileHandling), the service correctly issues the token. The request I am sending is as follows:

<SOAP-ENV:Envelope xmlns:SOAP-ENV = "">schemas.xmlsoap.org/.../" xmlns:tem = "">http://tempuri.org/" xmlns:aras = "">schemas.datacontract.org/.../Aras.AuthenticationBroker">
  <SOAP-ENV:Body>
    <tem:GetFileDownloadToken>
      <tem:param>
        <aras:fileId>21C0CBAAC7D0416DB58559C386841678</aras:fileId>
        <aras:dbName>InnovatorSolutions</aras:dbName>
      </tem:param>
    </tem:GetFileDownloadToken>
  </SOAP-ENV:Body>
 </SOAP-ENV:Envelope>

The response in case of error is as follows:

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:soap = "">schemas.xmlsoap.org/.../" xmlns:xsi = "">www.w3.org/.../XMLSchema-instance" xmlns:xsd = "">www.w3.org/.../XMLSchema">
  <soap:Body>
    <soap:Fault>
      <faultcode>AuthenticationBrokerValidation</faultcode>
      <faultstring>Unknown exception on AuthenticationBroker ---&gt; Object reference not set to an instance of an object.</faultstring>
      <detail/>
    </soap:Fault>
  </soap:Body>
</soap:Envelope>

What could be the reason? Thanks in advance.