Hey @Simeon
We gona need a lot more info Is the 400w coming from a qb configuration with all the diodes in one place that's directly above the plants (how many plants?? how far spread out?) if a bar style configuration is in use with the diodes spread out far and wide and no concentrated focus point where light is coming from it may all have an impact on the answer.
what I see + what you saying = It may actually be light, cause 400w at a distance of 28cm for seedling/todlers is overkill for sure, even from a bar light - but what do you mean normal LED? Is it a floodlight? not a growlight?
Gona give a long answer PAR on the leaf surface is what we focus on and it's measured in micromols per second, when measuring PAR at your canopy level you aim for round about 500micromol per second during seedling/veg stage moving all the way up as you hit late flower maxing out at just over 1000micromol per second, and then obviously as PAR increases the Kelvin Spectrum decreases, starting with higher 5000 to 6000K (white light) during seedling/veg stage and lowering all the way down to 3000K (yellow light) during flower. the reason for all this is the plant absorbs different light differently at different stages throughout its life. meaning, more light isn't always good, sometimes you got just the right amount of light but it's the wrong kind, like the spectrum is out and and and... so you pumping 400w (which is already too much) of what Kelvin? may also be out.
To achieve around 500micromol (can be bit above or bit below) on your leaf surface you can still get away with round about 50w at your stage. around 5000k will be best. even a CFL with those specs will work for you right now.
I focus on the light first, cause that's what you asked about, but with all that was just said, I actually saw something completely different right off the bat. Cannabis plants are so much more flexible with their environmental conditions above ground that you can really manipulate and do all kinds of crazy stuff with them above ground, as long as below the ground everythings as it should be. You can just raise your light a little and all should be fine in that regard, IF everything below the soil is fine.
First gotta ask if it's an auto or photoperiod plant? Autos stress when you fart near them, any kinda stress will have greater impact.
I would say your stress is root zone related by the looks of it, usually the worst time to deal with achieving perfect moisture levels is during seeling stage. Big plant you can over water and under water - no biggy. With seedlings just a few drops of water extra or too few drops of water can be a killer, too long wet without dry period can be a killer, too little oxygen by the roots can be a killer...a good estimated 90% the time when a plant dies of pythium (root rot) is during seedling stage. at that stage we just call it drowing/suffocating a baby. pythium happens a lot in hydro setups that have poor oxygen in the water, but in all cases seedlings are more susceptible to any kinda undesirable conditions.
Anyway, looking at your photos, it looks like you're in pure coco? correct me if I am wrong here. It seems completely saturated.
Back to - we gona need a lot more info - what's your watering regime? what kinda water you using? what are your other environmental conditions like (eg. FAE allows stable RH for plant media to not stay saturated for too long, if you water and plant stays wet for a week something is wrong) do you let the soil dry before watering? do you use nutrients at all and what kind? (synthetic / organic) do you water/feed till runoff? do you pour the runoff out or let the plant soak up runoff?