Intermittent proxy errors
Incident Report for Decisions on Demand
Salesforce has resolved the issue as of 21:38 UTC:

All Decisions on Demand servers are fully operational again. We appreciate your patience and apologize for any inconvenience caused
Posted Oct 16, 2015 - 02:17 UTC
Update from Salesforce:
The Salesforce Technology Team has identified a fix for the issue causing intermittent errors with Single Sign-On logins and outbound HTTP calls. The fix is currently undergoing testing prior to deployment. Deployment will begin once the testing and validation is completed. Please check the status of frequently for updates regarding this issue.
Posted Oct 15, 2015 - 20:45 UTC
The issue has been identified by Salesforce and a fix is being implemented.
Posted Oct 15, 2015 - 18:28 UTC
A fix has been implemented and we are monitoring the results.
Posted Oct 15, 2015 - 18:28 UTC
We have become aware of an intermittent issue affecting callouts from Salesforce to external services, including Decisions on Demand. You may see messages such as "Client side exception: Unable to tunnel through proxy. Proxy returns "HTTP/1.0 500 Internal Server Error"

Salesforce is working to resolve this issue and we are monitoring closely.

More details at:
Posted Oct 15, 2015 - 18:27 UTC