I am having the exact same issue in both CS6, and CC. I am using the latest data bridge (1.18) and the latest e-cart (6.04).
I have tried each of the preview choices (classic ie, webkit, web shot and web shoter), and none of them will show a preview in the preview window.
I have tried toggling off preview (then it correctly shows preview disabled), and re enabling it. It still will not show a preview in the preview window.
When changing a form element or design there is the progress wheel for a second or so, then instead of showing the preview, it just says click to preview in browser.
Any suggestions?
{edit} Interestingly enough, I saw there was a new version of ecart just released today that I hadnt noticed (6.05)... I installed that, and my preview is back, at least in CC. I will do the update in CS6 and see if it works there too.