Can't modify Desktop Behaviour

Hello all Trinity users, On one machine (a T400), recently installed with Wheezy 7.7 and TDE 3.5.13.2, I can't change the Desktop Behaviour. If I unselect "Show icons on desktop" they stay. If, in Device Icons, I choose "Show device icons", the screen goes black, then comes back unchanged (and the options have also been set back). kdesktoprc seems ok, same as any other install. It's not very important because that's not a main machine but I'm puzzled as to why this happens. Thierry