Login screen shows whitespace

[Somelauw]

I'm using lmde-xfce 64-bit and I just updated to upgrade pack 6.
If I enter my username and password at the login screen right after my computer has started up, whitespace (about 2 spaces wide) is shown instead of the letters/stars that I should see.
After I press one of the buttons like 'change session', my input becomes visible again.
Also, if I have logged in and then log out again, my input becomes visible again.
It seems that something is not loaded yet, that gets loaded as soon as I press "change session" or when I login.

It is not a very big issue since I can still log in (and type my name blindly), but I'm surprised no one has mentioned it yet before and I wonder if I am the only one with this problem.

[zerozero]

what graphic card and drivers are you using?