So I was having this problem where WordPress would display a blank editor for large posts when viewing visual mode. I could get it to work by using the HTML view and refreshing the page, but never in visual mode.
Anyway turns out to be the result of some modest PHP pcre regex defaults. Thankfully all it took was changing a few setting in PHP.ini. A bit thanks to the following blog post, which turned a nightmare of a bug to find into a quick and easy fix.