2 diagnostic LED, fan turning, no sound, no display after SSD and RAM

I’ve upgraded my 2017 Retina 4K two times already. Now I changed the RAM and the Blade SSD, cleaned everything and refreshed the grease on the CPU, GPU. After the reassembly it doesn’t start.

Black screen, no boot sound, the first two diag LEDs are on, every connections double checked.

The same happens without the SSD or with the previous RAM set.