After installing the upgrade you are supposed to restart your application to finish the upgrade process. What happened was that the whole edit window was totally blank. Well not totally I could see the colored edit bar, and everything that is rendered wen it's expanded. Admin mode also worked.
The problem was not related to the Preamble property error that Erik pointed out.
The way to resolve this was to empty the ASP.NET temporary files folder (located at C:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\root or Framework64), do an iisreset and start the site.
This is one of the things to take into account when deploying an upgrade to your different environments.
By the way the folders affected by the update are: