I asked some repair shop to look at the issue of the screen staying dark. With a flashlight they saw the LCD was getting data.

There were two issues, one caused by the other: A gold pin of a cable was bent. Levels on the mainboard were questionable, caused buy the shortcut made by the bent cable.

They bent the tiny gold pin on the cable back. I don't know if the shortcut solved itself that way.

They stated: Apple tells the power cable is never the issue, mostly something on the mainboard.

At the time of putting the datacable in the second time it was a bad lighting situation and someone working on my nerves in the room. It didn't go completely smooth, I had put it a bit towards one side.

So if that happens, draw back, try to look at the tiny pins and retry before continuïng.

This repair has cost me €118,20

The second time I reconnected the display the backlight and the display stayed dark and only 3 out of 4 leds lit up. I chose to bring it to a repair store to investigate whether only a cable needs replacement or just to recycle the whole machine. The boy who took in the iMac Mid2015 told me he didn't risk this internal replacement SSD himself at home, but had plugged in an external thunderbolt SSD instead to run the OS from. If I had thought of that before...