I am guessing your using Internet Explorer 8.0; I have seen this on some pages inside the 4.5 dashboard . Some times page elements are not displayed. Try this, logout of the console. In the IE window go to Page on the right hand side of the browser and choose Compatibility view. Once the page refreshes now login and attempt to create the on-demand-scan task.
Thanks a lot it's working with IE and compatibily view...
I was using Firefox with epo.
A weird thing is, on student's network I can create on-demand scan task without any grey screen with firefox..