Kinda late on the reply, but damping off usually has a very brown section and is wilted like this..
If you look at your stem, is it brown at all? It looks shiny where it is folded in half, it's kinda tall and I'm wondering if it got top heavy and folded over?
I have not been following this thread last few days so I don't know how close your lights are, but you may want to get them a bit closer to shorten up the stretch.
If it is damping off, it is fungal so you will want to dispose of all affected seedlings and clean well. You can use a fungicide either purchased or homemade if this becomes an issue that spreads.