Ideally the buildup should have been...
cladding
battens
membrane
studs = with insulation
internal cladding
Cladding nearly always leaks, either through butt joints, capalirary action or at corner details. Now your probably going to get someone saying "ah yes, but they build boats from cladding and they don't leak", but boats are lathered with paints and sealants, and because the timber is wetter, it stays tight up against each other.
So you can either lather it with sealants at the joints and paints all over, or you can put in a cavity.
If you can't put in a cavity, i would recomend putting in 19mm battens to the cladding side, and then some rigid insulation (or a board material and mineral wool), this means that there is then a space between the cladding and the insulation, and the insulation wont act as a wick for moisture.