So are we supposed to be using JsonP in order to call methods that require posts, since our apps aren't on the same domain? I'm having the same issue trying to add/remove favorites. I could swear it worked once on a get request, but it's not working now.

I just ran into this issue myself. I was also running into a secondary issue. I think I figured out what was causing the second issue but I'm not sure if it's a bug in the SE.authenticate() flow (using the SDK), or working as intended. Basically, if I didn't re-run SE.init() every time I tried to use SE.authenticate() within a single session, the proxy URL would return "undefined" at the end. Now that I figured out how to get around that, I still run into the issue mentioned here.