Um... what did you change right before it starting being what you don't want it to be? That'd be the thing to un-change!

Most people don't change the admin skin, or if they do they don't share how they do it in these forums. The best I can say is to click the "L" link in the top-right section of your admin area to get to the "legacy" skin.

The change was made about 3 versions ago and I don't remember what I modified. It have to be something in the database now, because all the files have been updated to dev 2

It's the "Admin skin" setting in your admin profile. Just click your username at the top of the page and you should be able to change it there. Already with version 1.8.x.

But: what are the problem's you've discovered with the new one?