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.

So I changed in ~/.gnupg/gpg-agent.conf:

- pinentry-program /usr/bin/pinentry-gtk-2
+ pinentry-program /usr/local/bin/pinentry-gtk-2