after a few tries I encounter the same problem. After the creation of the virtual copy for the proof, all sliders are greyed out and cannot be moved anymore. But, when switching to grid-view by pressing G, I see the virtual copy for the proof. After clicking on it and changing to the development module again, I can do the proof and use all sliders.
Sonds a bit complicated and should be changed, but it is a work-around.
But, I'm aware that I use the release candidat 5.2 and it is normal that these kind of version can contain bugs.