Go and literally stick your finger in the soil-is it wet or damp? yes? Your over watering and need to water less and less often.
Is it dry ?? Then water them more often with less water.
AND make sure your humidity isnt too high, no more than 75% at this stage.
The cause is overwatering. Do not water closer than 5cm (2 inches) near the plant. The roots need oxygen! When you water more to the outside of the pot the roots have acces to enough oxygen. Cannabis plants, especially young plants love to search for water! While doing this they create a larger and healthier rootsystem.