The attached sample project reproduces the following issue:

A HTML page tries to display a video served by a custom QWebEngineUrlSchemeHandler implementation. The video cannot be displayed correctly. While we see the request coming and, and can verify that the correct mp4 file is served with the video/mp4 mimetype, the developer console only shows us a canceled network request for that file. It is not clear who or what is canceling the request. Note that this approach works fine to serve static images, but fails equally when serving e.g. a mp3 file for audio. Accessing the multimedia contents via a file:// URL works though, so this clearly has something to do with the custom url scheme handler.