The funny thing is that I had corrosion going one everywhere but NOT on the membrane unit itself. It seems you had a different cause than I have. My bet for now is on the weeping endcaps of the high pressure pump and T-joint on the pre-filter. But it’s good to hear you now have no corrosion problems anymore.
By the way; don’t take Lagoon as any form of reference. Their only standard is (low) price. I do agree the spare parts of Dessalator are silly, but most parts are available elsewhere as well. I bought new (identical) membranes for approx 160 Euro each in the Netherlands.