Comment 1 for bug 284577

Also observing the same issue as above, though with a different:

KeyRelease event, serial 34, synthetic NO, window 0x5800001,
    root 0x1a6, subw 0x0, time 3980942, (-276,455), root:(463,515),
    state 0x10, keycode 174 (keysym 0x1008ff15, XF86AudioStop), same_screen YES,
    XKeysymToKeycode returns keycode: 164
    XLookupString gives 0 bytes:
    XFilterEvent returns: False

In the kubuntu.xmodmap I find:
 keycode 164 = XF86AudioStop

(I did receive exactly the same error as the poster above, but due to me having a play around it has changed, though how the new number 164 appeared, I am unsure, unless there is a newer version of the file (I reinstalled kubuntu-default-settings after changing XF86AudioStop to point to keycode 174.