Logon screen distorted (when display dimmed and TM has run)

Hello,

I have the following "problem" with my MacBook Pro with ML 10.8.1.

About once a week I leave my MBP on over night to let Time Machine backup my MBP (to an external USB drive).

The display is configured to dimm after 5 minutes, the MBP to 'never sleep'.

The next morning, when I press the SHIFT key to 'wake up' the display from the dimmed state, nothing seems to happen (screen stays dark).

However, when I move the cursor (moving my finger over the built in trackpad), little squares appear where the cursor moves over the screen.

Through the squares the normal background of the logon screen is shown.

As soon as I click on my logon icon, the password prompt is shown and the upper right quarter of the screen shows the background of the logon screen.

The other three quarters of the screen stay as they are.

When I have entered my password and press Enter, the normal background, dock, open windows etc. are shown.

The MBP then works normally.

Anyone having the same problem?

And, more important, has a solution for this?

Further information: I installed Lion from scratch (Internet boot) on a new blank hard drive a couple of weeks ago, then upgraded to ML re-installed the Apps I bought from the App store and restored my personal data from my TM backup.

Kind regards,

emeu1