Which login form are you referring to? I assume you are referring to the wizard "TMS XData Web Application", which creates a sample application with CRUD forms based on a database?
Such wizard doesn't provide login capabilities to the app.
In any case, can you please provide the exact steps to reproduce the issue?
I finally know why this is happening. I am getting the same effect when I use a MessageDialog on
a CRUD Edit form. A dialog box pops up, but is not visible.
It puts a fade block on top, which makes it look like its frozen. Followed by the dialogbox with a higher z-index.