I got the following error while trying to send a signed (GPG) e-mail using claws-mail:
Signature failed: Data signing failed, General error
Turns out it was a path problem. The GPG agent uses pinentry to ask for your private key password. The agent was configured with the absolute path to pinentry-gtk-2 on Linux. But this happened on FreeBSD and executables are located in /usr/local, not /usr.
Today I noticed that one of my PGP private key just disappeared of GPG. The key did not appear when I did gpg --list-secret-keys. After a bit of investigation I discovered that the problem did not affect Linux hosts but only FreeBSD hosts. Weird…
The source of the problem was a migration from GnuPG v2.0 to v2.1. According to this page, GPG does not handle the private keys anymore and delegates all private keys operations to the gpg-agent. Therefore GPG v2.1 migrates the legacy secret keyring, secring.gpg, to the gpg-agent key store, private-keys-v1.d and then forgets about it.
Though, you see, my GPG keyrings were synchronized across all hosts. But the GnuPG package on Debian is still v2.0, while FreeBSD is v2.1. Get the picture?
I synced my keyring on FreeBSD hosts where GPG migrated my private keys to the gpg-agent key store. Then I generated a new key pair on a Debian host, which was added to the legacy keyring. Resynced, but the newer version of GPG didn’t care, they already migrated to the new key store.
Fortunately it was easy to fix, all you have to do is re-import your legacy keyring with one of the newer versions of GPG. The private keys are now also present in the new key store so you can sync to all other hosts.