Hi Brendan,
I have seen this behaviour before release, I think there was a PHP warning involved. Probably there is still one there that only comes out when certain settings are set.
Is it possible for you to install the plugin Query Monitor, and see if there are undefined variables?
There should be a PHP warning in AJAX when you hit Preview from the frontend.
The information in Quey Moinitor is visible from the admin toolbar in the top.