Failure to sign with gpgsm

[Pedro Pessoa]

Altough I can sign with a certificate from Thawte, when using a certificate 
from the Portuguese nacional laywer association I'm having this error:
gpgsm: error creating signature: No value <KSBA>

The certificate tree is correctly verified:
gpgsm: DBG: gcry_pk_verify: Success
gpgsm: certificate is good
gpgsm: DBG: got issuer's certificate:
gpgsm: DBG: BEGIN Certificate `issuer':
(...)
gpgsm: DBG: gcry_pk_verify: Success
gpgsm: error creating signature: No value <KSBA>

Any thoughts on this? What's going on?

I've tried the following versions:
gnupg2 1.9.16 with libksba 0.9.11
and
gnupg2 1.9.22 with libksba 0.9.15
both give out the same error.

Thanks a lot!
Pedro