Making Video Play by Click

I’m trying to build a “clickable” screen, which – after you click it – plays a mp4 video. One of the example projects does exactly that… so I pretty much just copied the puzzle layout.
Yet when I put my link ./myvideo.mp4 into the field for the link it just shows a black screen after clicking it. But if I enter the link which is used in the example project the video starts… no black screen. Any Idea what I’m doing wrong?
(It does also not work when I use any ./xx.mp4 example file from the web)

(I turned readable etc. on for the file on my server)