Power Profiler on Linux - white screen

I have problems using the Power Profiler Kit from nRFConnect v.2.6.1 with a nRF52832 DK on Ubuntu Linux 18.04 (KDE Neon). It is unreliable in opening the serial port, but the main problem is that after a short while the screen of the application turns white and nothing more happens. Sometimes I can Start and see some measurement data before it turns white, sometimes it turns white even before I can press the Start button... I tried it already on a PC and on a Laptop, both with the same SW Versions.

Screenshot (of the white screen) is attached