Dried Seedling Leaf Edges

Hello,

I have some cucumber seedlings growing on a sunny window sill and the edges of the first false leaves have started to turn brown and dried along the edges. This also happened a few weeks ago to some cauliflower which were in the same location and it ended up happening to the real leaves as well and the majority died.

I wondered if they might be getting scorched from the sun through the window, but I did not have this problem last year when growing seedling in the same place.

Can anyone help determine what is causing this then I can counter it before these cucumbers get worse.

Thank you