Eu o consertei seguindo estas instruções de enigmail.net :
Check the contents of
$HOME/.gnupg/gpg-agent.conf. Make sure that there is a configuration entrypinentry-programcontaining the full path to a graphical version ofpinentryas above. E.g.:pinentry-program /usr/local/bin/pinentry-gtkThen save the file.
Add the following line to your
$HOME/.gnupg/gpg.conffile:use-agentThen save the file and restart your PC.
Estou no Ubuntu 14.04.4 LTS.