My CMS has stopped decoding Json so that to see a CMS page one must always reload after clicking a menu. If you do not reload the source of the page looks something like this:
{"Content":"<div id=\"pages-controller-cms-content\" class=\"has-panel cms-content center cms-tabset CMSPageEditController CMSMain LeftAndMain\" data-layout-type=\"border\" data-pjax-fragment=\"Content\" data-ignore-tab-state=\"true\">\n\n\t<div class=\"cms-content-header north\">\n\t\t\n\n\t\t<div class=\"cms-content-header-nav\">\n\t\t\t<div class=\"breadcrumbs-wrapper\" data-pjax-fragment=\"Breadcrumbs\">\n\t<h2 id=\"page-title-heading\">\n\t\t\n\t\t\t\n\t\t\t\t

After the reload you get HTML.
This is a major pain. What could be causing it?
I have a custom SiteCofig and one or two custom ModelAdmins but these are not complex things.
Where can I go looking for an error?

[edit] Currently using Version 3.2.1 of the Framework and CMS

For the benefit of the next person who runs into this problem:
A single tab before the PHP open tag in a ModelAdmin extension class file caused this.
While it was simple enough to discover this error it was somewhat arduous and debugging would have been much simpler if the error were reported somewhere.
As it is I had no way of knowing where the failure actually happened and I had no way to trace it back to the ModelAdmin extension class other than by following my nose.