You say the seedling gets very hard and dry, your temps and humidity sound ok so logically i'm thinking maybe it's not able to take up the water it needs? Is there enough moisture in your rockwool? I've never used rockwool but i remember seeing something on here about a few issues with it regarding it holding too much/little moisture.

Hopefully someone more qualified to help will be along soon

In the meantime i'd double check your rockwool procedures (soaking, watering, ph etc) and see if you can spot something

Good luck!