The name says it all, if you use NextGen Gallery and have problems with GD thumnbnail generation, you might want to consider upping the PHP memory limit. You probably already suspected it, and it can be done by simply uncommenting one line of code.

Link here: Thumbnail Generation PHP Memory Limit NextGEN Gallery » Visser Labs.

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.

Sudden empty / blank page for large posts with WordPress | Under My Hat.