What is your operating system?
Ubuntu 20.04.1 LTS
What is your Shotcut version (see Help > About Shotcut)? Is it 32-bit?
Can you repeat the problem? If so, what are the steps?
I created a new project and embedded a video file in it. I dragged the file to the timeline while the application was making proxy of it. The playback went fine. I closed the application without saving the project. Then, i reopened the project and dragged the same video file to the timeline. The playback is without sound and pictures with black background. The app shows that proxy is in use. The proxy file itself is fine if checked outside the application. This only appears every time I reopen the project that has the proxy already.
However, for the first time I met this problem was just after turning off the visibility and sound of a timeline track and later back on again which resulted with any newer track to be blacked out while only the original that wasn’t proxy to be played properly.