Prbably caused by you having the humidifier pointed directly at the plants.
If the xylem is completely rotted, the plant will die.........the dying leaves may point to this being the case already.
Tie some strings/twine from the ceiling down to the plant to support it and stop it leaning..........if you are lucky, it may pull through.........but you will need to lower the humidity big time while it recovers......as in, turning off the humidifier.
I would not treat it with anything, keeping it dry is essential.........it is now a question of how much this auto wants to survive.
If you still have spare seedlings, replanting might be the best option.
BTW, seedlings do not need to be in clone boxes/humidity domes...........it can cause stem rot........