Red pixel line down the LCD. Who can repair @ Berlin?

You can use a pencil and a microfiber cloth on the display where the issue is and place the cloth on the screen. Use the pencil's eraser and push the screen lightly increasing pressure slowly and then moving it over the affected area. Then go back and continue this process from different directions. Be firm but gentle.

I had exactly the same problem with a vertical red line in the far left side of the screen. It doesn't show on a white background because white calls for all pixels of R, G and B to be lit, so the black background is where you see it the most.

For one full line of red pixels to turn on it has to be either the graphics software or a kind of shortcut or missing connection that theoretically can occur within a processor or somewhere in the connected peripherals. I can't say with certainty, but it would be more logic for a display defect to manifest as missing pixels.

However, after a week or so where I was doing some work that led the Mac to heat up to full fan (of cause it cooled down a few times at night), and the line went away without a trace by itself and hasn't shown up again (problem occurred three months ago)

Maybe the temperature change cured it - Worth a try if you run into it.