Comment 2 for bug 152275

I am working on a similar issue.

Can you try this ?

1) From the top menu bar go to the System > Administration > Login Window
2) Enter your password when/if prompted
3) In the Local tab, make sure the Show Actions Menu box is checked,
4) Click "Close" to accept any changes
5) Logout
6) Login again
7) Press the power button or go to System > Quite and verify if the buttons are back