Same here with official Linux client 2.15.3, on the web interface, video shows without problem.
I noticed, but, the “player” is not the same on the Electron Client and on the web browser (Firefox)
This is the non-fullscreen player on the electron linux client
See the controls? On the first one, the full screen icon doesn’t do its function, and the three dots menu let download the video. On the second one, the maximize icon works, but there is no option to download the video.
That makes sense as the player on the first one is the embedded player on the electron client while the second player is Firefox’s embedded video player.
I finally installed chromium, which its player has the very same appareance as the RC Electron client, with the only difference when you hover the cursor over the controls, a shadow appears, but full screen works just fine.
So, I’m afraid it’s a RC desktop problem.
PS: Videos are MP4
PS2: Forum doesn’t allow me to upload more than one image, so I had to remove the chromiun one, just imagine the shadow when overing the cursor or try it on chrome, I guess it will behave like in chromium