I just did a fresh install of Ubuntu 12.04. Initially things were working. But after I installed some software, the 'gpg agent' is unresponsive. I suspect it has something to do with upgrades that I downloaded from the gnome 3 ppa. When I try to sign a package, it terminates with:

gpg: problem with the agent - disabling agent use
debsign: gpg error occurred!  Aborting....
debuild: fatal error at line 1271:
running debsign failed

The GPG gui tool (called "Passwords and Keys" or seahorse) isn't starting anymore either. When I click it, it tries to start and then gives up and dies after a couple of seconds.

I am not sure where to look for log files of gpg agent. The only thing that I see in /var/log is in auth.log that says:

May  1 20:04:14 jeroen-ubuntu gnome-keyring-daemon[1997]: couldn't create prompt 
for gnupg passphrase: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The 
name org.gnome.keyring.SystemPrompter was not provided by any .service files

Not sure if it is related, but when I try to start seahorse from the command line, I get:

jeroen@jeroen-ubuntu:~$ seahorse

(seahorse:4828): GLib-GIO-ERROR **: Settings schema 'org.gnome.crypto.pgp' is not installed

Edit: I fixed the seahorse GUI by manually downloading and reinstalling gnome-keyring version from precise instead of the ppa. However, I still cannot sign packages.