Hi,
I made a new simple project, this is what I did:
-Imported video: ShotCut asked to convert to constant frame rate, which I did.
-I exported the video and started a new project with the exported video:
-I added a simple intro (black image)
-exported the video (lib264 - quality 59%)
-the resulting video shows the intro image, but not the video: every frame in the playing video is just black