Thanks! We've been able to confirm this is an issue where Pro conflicts with certain optimization techniques introduced by the server or a plugin. Basically Pro needs to inject some information into that preview frame and expects to find a specific variable, but it's being altered some. We've made some changes in the next release to help Pro identify if this external change happens.
I've installed a patched version on your site and now your content can be edited. When the official release is available you can safely update.