The byobu version ( 5.74) which is included in Ubuntu 14.04 (Trusty Tahr) is broken. When you will use the escape sequence (ctrl-a) the session will freeze and you will have to kill the screen from another shell.
A fix exists but is not in the official ubuntu repositories. In order to install the fixed version you have to add the byobu PPA, update your sources and install it, i.e.:
sudo add-apt-repository ppa:byobu/ppa
sudo apt-get update
sudo apt-get install byobu
(The complete instructions are at byobu ppa)
However take notice that if you have already encountered the problem you must also reconfigure byobu else the problem will persist.
To reconfigure byobu, run:
then in the menu select ‘Escape Sequence’ and in the submenu just select apply and then exit.
byobu-screen should work now
The bug was fixed since 23 April 2014 in this commit byobu-trunk-ctrla-fix