I have had the Wordfence plugin installed for a good 6 weeks and in the past 3 weeks I have had the same issue. Deactivating all my plugins and re-activating one at time has shown that the Wordfence WordPress plugin is the issue with the blank page with a 200 status instead of the 302 redirect to wp-admin.
Maybe they are due an update soon or something clashes with another popular plugin that's recently updates, such as Jetpack.
Deactivating or uninstalling the Wordfence plugin fixes the problem. I know it doesn't tell you why it's all gone a bit off kilter but at least it confirms your Wordfence suspicions :)
If I discover any more I'll report back!