Numlock key not working in combination with specific layout (neo2)

I'm working with a fresh install of ubuntu 14.04 on my notebook (Samsung Chronos 7, 700Z5C-S02) and try to setup my default keyboard layout, which is German(Neo2).

When this layout is selected, I cannot use the Numlock key. If I watch the keyboard layout chart I see that the correct key is recognized, but the state of Numlock does not toggle. When switching to another layout, Numlock can be toggled normally.

Also, if I activate Numlock and then switch back to German(Neo2), I can use the number keys on the numberpad normally.

I did some research and found many suggestions of installing numlockx. But when I have selected the German(Neo2) layout, the program fails to start:

X Error of failed request: BadValue (integer parameter out of range for operation)
  Major opcode of failed request: 132 (XTEST)
  Minor opcode of failed request: 2 (X_XTestFakeInput)
  Value in failed request: 0x0
  Serial number of failed request: 17
  Current serial number in output stream: 20

Is there a way to get the Numlock key running with my layout?

I did some further testing, and found out that everything works fine if I install ubuntu on a virtualbox (Windows 8 as host) and setup the keyboard layout there. So the problem really only occures on my native install.

Also I thought it might be a problem that I have a US keyboard type, but there is no way for me to test with another keyboard.