Dear Norbert,
Sorry for not responding earlier. I had no usb keyboard at home so I waited till today to do it at the work.
I confirm that the PrintScreen USB Keyboard button works well. 
Here are the results for the USB Keyboard testing:

<PrintScreen>

Outer window is 0x4400001, inner window is 0x4400002

KeymapNotify event, serial 24, synthetic NO, window 0x0,
    keys:  4294967223 0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   
           0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   

KeymapNotify event, serial 24, synthetic NO, window 0x0,
    keys:  4294967223 0   0   0   0   0   0   0   0   0   0   0   0   8   0   0   
           0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   

KeyRelease event, serial 25, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 2870503, (-614,399), root:(157,451),
    state 0x10, keycode 107 (keysym 0xff61, Print), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False



<Alt+PrintScreen>

Outer window is 0x4400001, inner window is 0x4400002

KeymapNotify event, serial 24, synthetic NO, window 0x0,
    keys:  4294967223 0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   
           0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   

KeymapNotify event, serial 24, synthetic NO, window 0x0,
    keys:  4294967223 0   0   0   0   0   0   0   1   0   0   0   0   0   0   0   
           0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   

KeyPress event, serial 25, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3022739, (-347,336), root:(424,388),
    state 0x10, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 28, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3023003, (-347,336), root:(424,388),
    state 0x18, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeymapNotify event, serial 28, synthetic NO, window 0x0,
    keys:  4294967222 0   0   0   0   0   0   0   1   0   0   0   0   0   0   0   
           0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   

KeyPress event, serial 28, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3023003, (-347,336), root:(424,388),
    state 0x10, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeymapNotify event, serial 28, synthetic NO, window 0x0,
    keys:  4294967222 0   0   0   0   0   0   0   1   0   0   0   0   8   0   0   
           0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   

KeyRelease event, serial 28, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3023003, (-347,336), root:(424,388),
    state 0x18, keycode 107 (keysym 0xff15, Sys_Req), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 28, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3023003, (-347,336), root:(424,388),
    state 0x18, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False



<Ctrl+PrintScreen>

Outer window is 0x4400001, inner window is 0x4400002

KeymapNotify event, serial 24, synthetic NO, window 0x0,
    keys:  4294967223 0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   
           0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   

KeyPress event, serial 25, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3088579, (-473,366), root:(298,418),
    state 0x10, keycode 37 (keysym 0xffe3, Control_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeymapNotify event, serial 26, synthetic NO, window 0x0,
    keys:  4294967222 0   0   0   32  0   0   0   0   0   0   0   0   8   0   0   
           0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   

KeyRelease event, serial 28, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3088819, (-473,366), root:(298,418),
    state 0x14, keycode 107 (keysym 0xff61, Print), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 28, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3088931, (-473,366), root:(298,418),
    state 0x14, keycode 37 (keysym 0xffe3, Control_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False



<Shift+PrintScreen>

Outer window is 0x4400001, inner window is 0x4400002

KeymapNotify event, serial 24, synthetic NO, window 0x0,
    keys:  4294967223 0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   
           0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   

KeyPress event, serial 25, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3149763, (-409,284), root:(362,336),
    state 0x10, keycode 50 (keysym 0xffe1, Shift_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeymapNotify event, serial 26, synthetic NO, window 0x0,
    keys:  4294967222 0   0   0   0   0   4   0   0   0   0   0   0   8   0   0   
           0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   

KeyRelease event, serial 28, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3149955, (-409,284), root:(362,336),
    state 0x11, keycode 107 (keysym 0xff61, Print), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False




<Ctrl+Alt+PrintScreen>

Outer window is 0x4400001, inner window is 0x4400002

KeymapNotify event, serial 24, synthetic NO, window 0x0,
    keys:  4294967223 0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   
           0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   

KeyPress event, serial 25, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3197625, (-606,426), root:(165,478),
    state 0x10, keycode 37 (keysym 0xffe3, Control_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyPress event, serial 28, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3197689, (-606,426), root:(165,478),
    state 0x14, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 28, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3198001, (-606,426), root:(165,478),
    state 0x1c, keycode 37 (keysym 0xffe3, Control_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 28, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3198001, (-606,426), root:(165,478),
    state 0x18, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeymapNotify event, serial 28, synthetic NO, window 0x0,
    keys:  4294967222 0   0   0   0   0   0   0   1   0   0   0   0   0   0   0   
           0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   

KeyPress event, serial 28, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3198001, (-606,426), root:(165,478),
    state 0x10, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeymapNotify event, serial 28, synthetic NO, window 0x0,
    keys:  4294967222 0   0   0   0   0   0   0   1   0   0   0   0   8   0   0   
           0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   

KeyRelease event, serial 28, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3198001, (-606,426), root:(165,478),
    state 0x18, keycode 107 (keysym 0xff15, Sys_Req), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 28, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3198001, (-606,426), root:(165,478),
    state 0x18, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False



<Ctrl+Shift+PrintScreen>

Outer window is 0x4400001, inner window is 0x4400002

KeymapNotify event, serial 24, synthetic NO, window 0x0,
    keys:  4294967223 0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   
           0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   

KeyPress event, serial 25, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3250511, (-466,397), root:(305,449),
    state 0x10, keycode 37 (keysym 0xffe3, Control_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyPress event, serial 28, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3250535, (-466,397), root:(305,449),
    state 0x14, keycode 50 (keysym 0xffe1, Shift_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeymapNotify event, serial 28, synthetic NO, window 0x0,
    keys:  4294967222 0   0   0   32  0   4   0   0   0   0   0   0   8   0   0   
           0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   

KeyRelease event, serial 28, synthetic NO, window 0x4400001,
    root 0xb6, subw 0x0, time 3250695, (-466,397), root:(305,449),
    state 0x15, keycode 107 (keysym 0xff61, Print), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False