I have just set up a new machine with Kubuntu 15.04, not re-using any KDE settings folder, and found that Alt-Tab does not work.  I suspect the problem to be related to my unusual keyboard layout (German Neo layout).

Maybe the product/component should even be changed to the global shortcut handler?

Other shortcuts work (except Shift-Alt-Tab for reverse walking), and I tried to change the shortcut, with the following results:
* Setting it to None and back to Alt-Tab does not work.  Alt-Tab is properly recognized and set by the shortcut widget, yet the shortcut does not lead to any action.
* (Shift-)Meta-Tab also does not work, even though it is recognized in the settings widget. (I confirmed to reassign from the activity switching actions. As expected, no reassignment dialog appeared for Alt-Tab.)
* Alt-X works (in the Neo layout, X is the key next to Alt, labeled Q on typical hardware).

This is the xev output for pressing Alt-Tab:

KeyPress event, serial 37, synthetic NO, window 0x8800001,
    root 0x293, subw 0x0, time 5181172, (-304,488), root:(478,512),
    state 0x0, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyPress event, serial 40, synthetic NO, window 0x8800001,
    root 0x293, subw 0x0, time 5181699, (-304,488), root:(478,512),
    state 0x8, keycode 23 (keysym 0xff09, Tab), same_screen YES,
    XLookupString gives 1 bytes: (09) " "
    XmbLookupString gives 1 bytes: (09) "       "
    XFilterEvent returns: False

KeyRelease event, serial 40, synthetic NO, window 0x8800001,
    root 0x293, subw 0x0, time 5181835, (-304,488), root:(478,512),
    state 0x8, keycode 23 (keysym 0xff09, Tab), same_screen YES,
    XLookupString gives 1 bytes: (09) " "
    XFilterEvent returns: False

KeyRelease event, serial 40, synthetic NO, window 0x8800001,
    root 0x293, subw 0x0, time 5182723, (-304,488), root:(478,512),
    state 0x8, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False


Reproducible: Always

Steps to Reproduce:
1. Set up Neo layout ("setxkbmap de neo", yet I configured it already in the Kubuntu installer).
!! You might want to prepare a second terminal with the setxkbmap command for switching back, because it probably becomes difficult to type with the Neo layout shuffling nearly all letters. !!
2. Observe that Alt-Tab does not allow to switch between windows.