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:
http://<your_virtualhost>/ez
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:
http://<your_virtualhost>/ez
You will get an error:

CAPIError: Invalid authInfo parameter           CAPI.js (line 457, col 27)
CAPIError.prototype = new Error();