A new candidate on the workbench: A Samsung Galaxy Tab which out of nowhere was showing a black screen. It is a Samsung Galaxy Tab A SM-T585. Plugging a charger would trigger a short vibration and a sound indicating that the device is obviously still working and just the screen is not doing what it is supposed to do. Luckily there is a ton of documentation out there showing that this is a loose display connector in almost 100% of the cases. See [1] for example. Opening the case is rather simple as it is just snapped into the back cover and can easily be opened using a flat plastic tool (e.g. iFixit toolset).
It is recommended to disconnect the battery while doing the repairs. This is the right flat connector if the tablet is upside down with the USB port at the top:
The display connector is the right (and slidly wider one) of the two in the middle. On the upper part of the PCB it secured with some tape (Which obviously does not do its job):
After removing the tape it is clear that this flat cable is obviously not connected properly any more. The white line on the cable is slightly below the golden obviously reference marker on the PCB. The connecting pads are partly visible:
So very easy fix here: Lift the black snap slider with a small tool then gently push the display cable back into the connector. The white line is now slightly above the PCB reference marker. After reconnecting the battery and pushing the power button for 3 seconds the display comes to life again:
Please be aware that this may eventually break your warranty. Use at your own risk. I am not liable in any way for damage to your equipment.