[SOLVED] Keyboard volume up working, volume down not

[Peiote]

Software
XBian version: 1.0 (knockout) (kernel: Linux 4.4.6+)
XBMC/Kodi version: 16.0 Git:2016-03-11-a3eb4fa
Overclock settings: I didn't overclock it

Hardware
Device type and model: Raspberry Pi 3
Power supply rating: 1,8 Amp
SD card size and make/type: 16 GB
Network (Ethernet or wireless): Ethernet
Connected devices (TV, USB, network storage, ...): Keyboard-> Microsoft Wireless Entertainment keyboard 700

Problem description:
Volume controls work fine in other raspberry pi systems. In XBIAN it works just "UP Volume".
Funny?