Most strains are going to struggle going in to systems that small. In the future I would hand water until you have at least three nodes showing with a real set of leaves.
Most will argue, but the Lucas formula was designed for flowering first and foremost. One can veg with it but it is not ideal. At that size all that was really needed was properly PHed water.
I'd still say that is too high. Closer to 250 would be better at that size but it may be too late for that one. Chances are the roots have been burned causing some rot.
As an experiment mix up your nutes and put them in something to store for a couple of days lid open. The PH should drop [that is normal for new nutes] and if they do not the clay was not cleaned quite enough which is causing the PH to rise. If it does rise, then something is off about your tap water and I would change over to some local drinking water if you can obtain that in 5 gallon jugs.
What are you using to drop the PH currently?
EDIT also, you want to remove that paper from the cube b 4 going in to the system. It can and will cause rot.