I noticed that the Temperatures were way to high for this young seedling, so I decided to turn my ventilation a bit up.
This way, around 24-25°C and ~40-50% humidity were reached.
At Day 12 I started with some LST as I need to keep the plant as low as possible. As the whole PC-Case is only about 45cm high - from which I loose around 10-13cm to light- and potsize - I need to use the space left efficiently.