Emacs doesn't set mark with C-<SPC> anymore after Xubuntu upgrade to 14.04

[Timothée Flutre]

I just upgraded Xubuntu on my laptop from 13.10 to 14.04. I uninstalled and
re-installed Emacs and its version is 24.3.1. Everything is working fine
except the kill-yank system (copy-paste).

To investigate this, I opened a terminal (xfce4-terminal 0.6.3), launched
emacs inside it (emacs -nw), wrote a few dummy lines in *scratch* and
kill-yank one of them (C-space C-n Esc-w). But when I entered the last
command ("Esc-w"), the mini-buffer displayed the following message:

The mark is not set now, so there is no region

I had no idea what was going on because this was working fine all the time
I was using emacs for several years. So I looked up online and found in the
manual (
https://www.gnu.org/software/emacs/manual/html_node/emacs/Setting-Mark.html)
that C-<SPC> was used to set the mark. Same for C-@. And indeed, the latter
works fine even though the former doesn't.

On StackOverflow, I learned that C-<SPC> was not working likely due to the
fact that the terminal emulator wasn't properly transmitting C-<SPC> to
emacs (http://stackoverflow.com/a/16271042/597069).

My questions therefore are: should I get use to C-@? Why was C-<SPC>
working before and not anymore? Is there something to add to my .emacs
profile to make C-<SPC> work all the time with emacs v24+?

Thanks for your help!

Tim