Hi I have the same issue.
I just upgraded and now the page is blank when I go to view my CSS blocks
I did the browser refresh, I have cleared the cache and I have deactivated all plugins and I have tried in another browser. (not all at the same time)
Still a blank page.
Running WP 4.0 on Linux
The weird this is I upgraded on another site , and it was blank at first, but displayed ok after a browser refresh.
This plugin is brilliant and my life and blood for many of my client’s sites.
I can’t upgrade them till my own site is ok.
Additional info.
Both of the sites is running the same theme – OptimizePress 2