Win 8.1 one of 4 USB-Monitors shots Startmenu

[Hunv]

Hi,

first see the attatched screenshot.
The big screens are not USB-Monitors. The 4 small ones are USB-Monitors.
On the left monitor at the bottom, there is always the startmenu (without the tiles). If click on this screen, the startmenu appears. If I click on desktop on the startmenu, the tiles disappears but the violet background stays (like on the screenshot).
I've got the latest driver (v7.4).
Any Ideas?