Steps to reproduce:
1. Ensure all caches are cleared, both ezpublish's and the browser's;
2. Access frontend first:
http://<your_virtualhost> (for instance)
and wait for everything to be loaded.
3. Open firebug or similar tool and go to Console;
4. Next, access PlatformUI:
You should be able to access it without a problem.
5. Now clear all caches (ezpublish's and browser's);
6. Access directly to platformUI, from a no-cache state:
You will get an error: