Error when calling BODS as a Web Service

I have created a Python process that calls a JOB from the data service (BODS).

I am able to call the authentication job, and it returns me a SessionID. Than with the session ID, I call a job service. Later I poll every X seconds this service to see if it has finished or not.

Sometimes, it works like a charm. The job runs and I am able to poll for the results.

MOST OF THE TIME, the server simply returns different problems (job not found, sessionID not valid, etc). Five seconds later, the process try again and it randomly works, with same code, call, sessionID, all.

Below an example of the error XML.

<?xml version='1.0' encoding='utf-8'?>     <soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">          <soapenv:Body>               <soapenv:Fault>                    <faultcode>Sender</faultcode>                    <faultstring>Web Services security enabled and session validation failed.  Error: Invalid sessionID supplied in soap request.</faultstring>               </soapenv               <soapenv:Fault>          </soapenv:Body>     </soapenv:Envelope>

Can someone explain me this instability? How could I check what is happening? Any log or evidence of why the error occurred?

Thanks