Hi,
Many issues cause this error. In my case it was the 'CERTIFICATE' issue. I got the same error when i was calling the web service from sales force. The reason i got this error was I did not load the correct sales force CERT in SAP under TCODE: STRUST.
Best way to find out is error is:
1. Goto tcode: SMICM
2. Menu goto->trace file>display all
3. scroll all the way to the end and find the error based on the time you ran the service call.
You should get some idea about what is going wrong. If it is the CERT issue, work with basis team to upload the certificate.
Hope this helps,
Karuna