No video shown in clip- and project monitor kdenlive

In the current version of kdenlive (17.08.3-1.1 from the openSuse repo) there is no video shown in both clip monitor and project monitor.
The clips are shown in the clip overview, the edited project is visible in the timeline (both audio and video), and I click “Play” in both Project or Clip monitor
but all I get is the audio.

The effect is the same in all my current projects, and also if I start a new one.

I have never had this behavior in the previous releases.

Anyone an idea what can cause this?