10.19. I get the message “OpenPGP error; Encryption/signing failed; send unencrypted message?”

This happen when you're writing a mail and you have enabled the option OpenPGP → Attach my Public key to send your public key with the mail. This option causes some problem due to a bug. As a workaround, choose OpenPGP → Attach Public key... instead, then in the Key Selection window select your key and click Ok.