Comment 117 for bug 221112

Same as Philippe. I forgot to mention that this is the output when the search in Rhythmbox works.
Here is the result if I switch back to the default option (but still using "French Alternative" layout). So that Rhythmbox's search doesn't work anymore and, as you can see, the space in Ctrl+Space is a space:

// pressing left Ctrl

KeyPress event, serial 36, synthetic NO, window 0x4400001,
    root 0x1a7, subw 0x0, time 19372022, (18,174), root:(1402,642),
    state 0x10, keycode 37 (keysym 0xffe3, Control_L), same_screen YES,
    XLookupString gives 0 bytes:
    XmbLookupString gives 0 bytes:
    XFilterEvent returns: False

// pressing space

KeyPress event, serial 36, synthetic NO, window 0x4400001,
    root 0x1a7, subw 0x0, time 19372214, (18,174), root:(1402,642),
    state 0x14, keycode 65 (keysym 0x20, space), same_screen YES,
    XLookupString gives 1 bytes: (20) " "
    XmbLookupString gives 1 bytes: (20) " "
    XFilterEvent returns: False

// releasing space

KeyRelease event, serial 36, synthetic NO, window 0x4400001,
    root 0x1a7, subw 0x0, time 19372358, (18,174), root:(1402,642),
    state 0x14, keycode 65 (keysym 0x20, space), same_screen YES,
    XLookupString gives 1 bytes: (20) " "
    XFilterEvent returns: False

// releasing control

KeyRelease event, serial 36, synthetic NO, window 0x4400001,
    root 0x1a7, subw 0x0, time 19372694, (18,174), root:(1402,642),
    state 0x14, keycode 37 (keysym 0xffe3, Control_L), same_screen YES,
    XLookupString gives 0 bytes:
    XFilterEvent returns: False

------------------ pressing only space : ------------------

KeyPress event, serial 36, synthetic NO, window 0x4400001,
    root 0x1a7, subw 0x0, time 19368942, (18,174), root:(1402,642),
    state 0x10, keycode 65 (keysym 0x20, space), same_screen YES,
    XLookupString gives 1 bytes: (20) " "
    XmbLookupString gives 1 bytes: (20) " "
    XFilterEvent returns: False

KeyRelease event, serial 36, synthetic NO, window 0x4400001,
    root 0x1a7, subw 0x0, time 19369046, (18,174), root:(1402,642),
    state 0x10, keycode 65 (keysym 0x20, space), same_screen YES,
    XLookupString gives 1 bytes: (20) " "
    XFilterEvent returns: False

So to sum up:
If you select the keyboard layout option to send a regular space (the workaround case), Ctrl+Space send a "00" event.
If you select the default keyboard layout option (the bug case), Ctrl+Space send a "20" event.