Hi,<br>
<br>
I found the answer here: 
<a href="http://askubuntu.com/questions/129781/issue-with-gpg-agent-in-ubuntu-12-04-after-installing-gnome3-shell">http://askubuntu.com/questions/129781/issue-with-gpg-agent-in-ubuntu-12-04-after-installing-gnome3-shell</a><br>

<br>
The cause is a missing dependency on the 'gcr' package, which provides the org.gnome.keyring.SystemPrompter service.<br>
<br>
A workaround consists in installing 'gcr' by hand.<br>
<br>
-F