I thought this was a plugin problem at first with wangguard and contacted them about it. Then I thought it might be Jetpack's problem and contacted them. I have now completely uninstalled wangguard, disabled jetpack's publicize, reinstalled 3.9.1, and still get a white screen every time I hit publish or update.
The page takes quite some time before taking me to the white screen too, as if it's looking for something it can't find.
If I hit reload on the browser, then hit the resend button, I can get back to the post edit screen. The post does appear on the blog, edits will appear, but this blank screen has been happening for the past week or more.
WangGuard was the only recently-added plugin and the only plugin I added since 3.9.1.