Hi.
I'm trying to do a little colour correction on a video taken on a mobile phone.

I created a new project, imported the video and added the curves filter.
However, the Project Monitor is black.

I deleted the project and started over. I noticed that right from the beginning the Project Monitor was black.
If I play the video then I can hear the audio, but there is no video output to the Project Monitor.

If I switch to the Clip Monitor, then I can see video (and hear audio). But the effect settings don't apply.
I'm assuming that maybe the effects can only be seen on the Project Monitor when it works. But it just isn't working.

What could be happening here?

Update and more info

I'm using Ubuntu 18.04

Initially when I had the problem I was using kdenlive 17.x.
I decided to update to the latest available version to see if that would help.

I updated to 20.04.0.

After the update both the project monitor and the clip monitor just show black.

Is this an issue with the software?

Further update

I purged kdenlive from my system.

I then reinstalled using the appimage build.
So now I'm not getting any error or warning about missing libraries.

However, the issue still remains - neither the clip monitor nor the project monitor show anything. They are both black.

Just like before: If I play the video in the timeline, I hear audio but there is no video output to the panes.

Just to be clear, the video that I included as a clip plays perfectly well in video players. I tested in totem and vlc.

Is there anything I can actually do here?