Shift + Ctrl + Backspace is a hot hotkey to navigate to "Last Edit Location".
Suddenly it stopped working on the left side, meanwhile the right side combination of the same keys work as expected.
Actually, I realized that left combination of Shift + Ctrl does not work at all. So Shift + Ctrl + Insert does not show the popup "Choose content to paste".
Further more I realized it's a system wide problem, in Google Chrome browser Shift + Ctrl + T supposed to reopen closed tub, Instead triggered just Ctrl + T which opens a net tub. I have Ubuntu 16.04.
I realized that it's an old well known issue with ubuntu. First I figured out that the problem rises when you use Shift + Ctrl to switch keyboard layouts. The issue is described here https://bugs.launchpad.net/ubuntu/+source/gnome-settings-daemon/+bug/1245473
Then It's downed on me that the real issue is that keyboard layout switching triggers on key press instead of key release. So this prevents any additional key strokes to be used with aforementioned combination https://bugs.launchpad.net/ubuntu/+source/gnome-control-center/+bug/36812.
Here is the solution:
https://habrahabr.ru/post/87408/
https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/1683383
In a few words, you need to apply a patch to the Xorg source and build xorg-server from patched files. For some reasons I failed to implement this solution and broke my xorg-server. However, when I reinstalled Ubuntu the problem has gone.