Running Ubuntu 15.10, 64 bit, Desktop, Unity, German keyboard layout, external PS2 keyboard.
I had this problem also on the same machine with 15.04, but not with 14.10, I think (both 32 bit).
After login, NUM-Lock is active and the LED on my keyboard shines. But very soon, it will go off, although the NUM-Lock state did not change, i.e. I can still type numbers on the numpad. But the LED does not match the internal state. It may go off randomly, but usually directly within the first minute(s) after login.
If I hit NUM-Lock now, the LED stays off and the internal state toggles as expected to off as well, so the numpad works as arrow keys now and both LED and real status match. Hitting the key again will turn both LED and NUM-Lock on again, as one would expect it.
Switching TTYs (e.g. to TTY1 and back to TTY7) also "repairs" the inconsistency and the LED shines again, as expected.
Is this random LED/real state mismatch a bug or how can I fix it?