Salesforce session expired:
- Check your API usage under Salesforce Setup > Administration Setup > Company Profile > Company Information > API Requests, Last 24 Hours.
- Ensure "Lock sessions to the IP address from which they originated" is NOT enabled
- Ensure the user running the solution has "API Enabled" on their Profile
- Ensure you have not reached your maximum API request limit
- Check if the server for your Salesforce instance is experiencing downtime or degradation issues. See http://trust.salesforce.com/trust/status.