I have two laptops with KDE Neon and Plasma 5.12.2 installed.

Both of the laptops are setup in exactly the same way, but only in one of them the Alt+Shift+Backtab and reverse task switching is working :/
In the second laptop the keyboard layout is changing when I press Alt+Shift+Backtab, and the forward task switching is activated instead of the reverse.

Anyone has some pointers to what I could look at in order to make the reverse task switching work with Alt+Shift+Backtab in both laptops?

You might confirm that the key combination is registered correctly in System Settings -> Gestures and Shortcuts -> Global Keyboard Shortcuts -> KWin, "Walk Through Windows (Reverse)". If it appears to be registered correctly but not working, you could try entering the shortcut as a custom shortcut just to see if the keyboard may be emitting a different sequence when you press that shortcut.

If I disable the "Alt+Shift" key combination for System Settings -> Input Devices -> Keyboard -> Advanced -> Switching to another layout, then the reverse task switching is working. As long as I re-enable the "Alt+Shift" key combination to switch to another layout, the reverse task switching is not working anymore :/

Trying to set any shortcut that uses "Alt+Shift+whatever" is impossible when "Alt+Shift" is used for switching to another layout. I'm puzzled since the exact same setup is working on a different laptop.

Yeah, I find it odd to have different behavior between the two machines with the same setup. Perhaps there's something different in the x input config somewhere which is different (one responding to the Alt+Shift combo on key press, one waiting to respond to that until release). Unfortunately I am not familiar with any of the internals there.

At the very least you can work around the issue by either disabling the layout switching picking a different combo which doens't interfere with the other shortcuts.

I'll probably live with it. Thank you for the suggestions anyway

BTW here is an unanswered question that mentions the same problem.
https://unix.stackexchange.com/question ... a-shortcut

I have the same problem. When I set shortcut for changing layout (alt+shift), then all other shortcuts that include alt+shift does not work.

Is there any solution or workaround other than removing the shortcut.
Is there any bug reported?

I've created bug: https://bugs.kde.org/show_bug.cgi?id=392961
please vote!