Kekon over at APC have made the conclusion that this can be caused by too much nitrogen in low KH all other things in unlimited supply in certain plants:
Ca, Mg, K, NO3, PO4 - mysterious summary - Aquatic Plant Central- aquascaping...a living art

I have followed Kekons progress since I have the exact same problems as him, and a copule of other low KH-people in Swedish and international forums.

I have killed all my fish with CO2 and all my snails with iron and all my shrimp with KNO3 and the pearling was close to none until I made a couple of 95% water changes and stopped dosing altogether. Then the pearling came back, until started dosing 10 ppm NO3, 1 ppm PO4, 0.1 ppm Fe where the curl-symptoms came back and the pearling stopped.

Low light, KH of around 2.5, insane amounts of CO2 (4 bubble/sec in 60 liters with mist and low pH - drop checker always very yellow).

The curling effect is less with large pieces of driftwood (other kind of buffering capacity?) and carbonate dosing...