eh, sorry but could you give some information? like used window manager(or overall desktop environment), what distri, kernel version, X version, drivers, etc.

EDIT: btw, I had similiar issues a while back with xfwm.

Desktop environment is Unity. I'm using Ubuntu. Kernel version is 3.8.0-31-generic. I have no idea how to check what my X version is and I didn't install any drivers on my own.
That's what I actually meant:

I can't just press ctrl+c, it won't work. I have to choose "copy" in context menu.

yeah, as I mentioned before, I had that with xfwm before. But you're using Unity which uses Metacity, so technically it should work :-/

now I'm not a Debian/Ubuntu user, so my knowledge about apt is not big, but I think you can get package information by typing "apt-cache show " and for you type xorg and so on.
Please also post your hardware specs.

This is what "apt-cache show xorg" says:
Version: 1:7.7+1ubuntu4

CPU: AMD Phenom II X6 1055T;
GPU: AMD Radeon HD 5870;
MB: MSI 890FXA-GD70;
RAM: 2x2Gb DDR3, something from Samsung.