A few questions:
“The new upgrade” of what? avast! or Vista?
Are you saying that all the other skins worked on Vista 5472 with previous builds of avast!, and stopped working after the program update? Did you reinstall avast! (I mean - if you didn’t, the skins installed previously should still be there, i.e. there shouldn’t be any need to download them again).

So, what are the skins that do work?

When exactly do you get the error - when you click on the skin (to install it), or when starting the Simple User interface subsequently? Or when browsing the skin selection dialog maybe?