I experience the problem, but no "error 18 at 0 depth" for me:
# openssl verify -CAfile <ca cert> <server cert> <client cert> <server cert>: OK <client cert>: OK
I experience the problem, but no "error 18 at 0 depth" for me:
# openssl verify -CAfile <ca cert> <server cert> <client cert>
<server cert>: OK
<client cert>: OK