I  have been working on a Force.com Canvas App integration between Salesforce and ADF. The Canvas App is OAuth and viewed within a Community on a Visualforce Page. Before Winter 16 upgrade, everything was working fine. Winter '16 was released for the sandbox over the weekend and now all Canvas Apps fail to render regardless of the Canvas App URL.

The error is "Unable to retrieve signed request from the server. Please try your request later."

In the DevTools console in Chrome, I see a network request to:

https://example.force.com/services/data/v35.0/platformconnect/signedrequest?canvas=Desktop_Canvas_App

In an org that is still on Summer '15, the Canvas App request is

https://example.force.com/SitePrefix/services/data/v35.0/platformconnect/signedrequest?canvas=Desktop_Canvas_App

The issue seems to be that the Site Prefix used for the Community is not being included in the request and an internal server error is returned and the Force.com Canvas fails to load and displays an error.

If I view the same page outside of the Community as a System Administrator user, the canvas app does load, but the communication between the Visualforce page and the Canvas App URL fails.

I have added all possible permissions to the Canvas App as well as revoked the Canvas App access and re-initialized the OAuth access to the app, but nothing works.