Hello,

Angel de Vicente <angel.vicente.garrido@gmail.com> writes:

> (add-hook 'message-setup-hook 'mml-secure-message-sign-pgpmime)

Solved by changing it to:
(add-hook 'gnus-message-setup-hook 'mml-secure-message-sign-pgpmime)

Cheers,
-- 
Ángel de Vicente                 -- (GPG: 0x64D9FDAE7CD5E939)
 Research Software Engineer (Supercomputing and BigData)
 Instituto de Astrofísica de Canarias (https://www.iac.es/en)