Mathematica crashes and falls into a ubuntu login screen when I click open

Hi,

I am trying to use Mathematica in Ubuntu.
It works fine, and I have not found any problem working with it except that
I cannot save a notebook because whenever I click the “save” button from the menu → file → save,
Mathematica crashes and my desktop falls into a login screen.
So, I guess what is crashed is not Mathematica, but probably GUI of the OS, say X.
The same behavior shows when I click “open” button, too.

I haven’t seen any other circumstances that gives me the same behavior other than Mathematica.

Before falling into the login screen, I could read an error message that quickly disappers, and it seems it is related to
vtsspp.

Anyone has any idea?