Part of the screen has no image after repair

The touch screen connector was replaced. After that, my device worked okay for about a month, until this issue appeared:

About one quarter to one third of the screen became gray blank. It is always the same size, but the “texture” changes. When you turn the screen on, this blank part starts black, then it slowly goes gray (brighter) and so the texture becomes clearly visible. Also, there are very rare moments, when this blank part disappears at all and you can see the full screen, as it should be, for a second or less.

What I’ve noticed recently (and I’m not sure it was so from the start) is that exactly this blank part is not responsive to touch either.

What might be the problem? What parts of the device have to be replaced or repaired?