Sorry you are having this problem. More information, please...

1) Do
 $ lspci -v
and post *just* the section headed "VGA compatible controller".
Is your system one of those that effectively has 2 graphics chips?

2) Do you have the 'tainted' media enabled? If not, enable them, update your system to apply them, and try again.

3) Are the graphics bad for other applications like image viewers or Gimp? Or is it just videos? And what about on-line videos?

CC: (none) => lewyssmith

1) There is only one graphic chip. Here the output of lspci:

00:02.0 VGA compatible controller: Intel Corporation UHD Graphics (rev 02) (prog-if 00 [VGA controller])
	DeviceName: VGA
	Subsystem: ASUSTeK Computer Inc. Device 1cd1
	Flags: bus master, fast devsel, latency 0, IRQ 132
	Memory at b0000000 (64-bit, non-prefetchable) [size=16M]
	Memory at a0000000 (64-bit, prefetchable) [size=256M]
	I/O ports at 3000 [size=64]
	[virtual] Expansion ROM at 000c0000 [disabled] [size=128K]
	Capabilities: <access denied>
	Kernel driver in use: i915
	Kernel modules: i915

2) Tainted media has been enabled for a while and I have updated the system several times.

3) There are only problems with video and screen recording. Gimp, image viewers and online videos are fine. 
There are much more problems when I am using the Plasma environment and for that I opened already a bug report a few weeks ago (#26399).

Thank you for the extra information.
Can you try other other video players (you have already tried Mplayer), like Totem (alias Videos), which is for Gnome; Parole. And report back.
---
Difficult to know where to assign this, start with drivers, CC Shlomi for his opinion; but note that mplayer is also affected, so it does not look like a VLC only problem.

CC: lewyssmith => shlomif
Assignee: bugsquad => kernel

(In reply to Béat E from comment #0)
> Description of problem: I made a fresh install of Mageia 7.1 on my new
> notebook Asus VivoBook 15 X512FA and updated the whole system. There is a
> problem with the display when I am using Plasma (bug #26399). I switched to
> Cinnamon Where the desktop works fine. But watching videos doesn't work
> properly. In VLC I can hear the sound, but see a black window instead of the
> video.
> 
> When I start VLC from the console I see the following messages:
> 
> VLC media player 3.0.8 Vetinari (revision 3.0.8-0-gf350b6b5a7)
> [00000000022e85b0] main libvlc: Lancement de vlc avec l'interface par
> défaut. Utiliser « cvlc » pour démarrer VLC sans interface.
> [00000000023c1280] qt interface error: Unable to load extensions module
> 
> I tried SMPlayer and there are problems too. The screen is black at first
> and I can see the video only when I switch to full screen and back. When I
> click on the video with the mouse the screen turns black. There is something
> more I discovered. Recording my screen doesn't work properly. With OBS
> Studio the sound gets recorded, but instead of the windows on the screen I
> see only a black wallpaper with the icons on it. 
> 
> This problem might  be related to my graphic card Intel UHD Graphics.
> 
> Version-Release number of selected component (if applicable): VLC 3.0.8
> Vetinari (but it affects also other software)
> 
> Steps to Reproduce:
> 1. Open a video file with VLC
> 2. There is sound, but no video (it is black instead of the video).

Please try mpv with the "--vo=[driver]" flag based on some of these:

https://github.com/mpv-player/mpv/blob/master/DOCS/man/vo.rst

Does it work?

Moreover, please use "export LC_ALL=C" before copying the error messages - see locales.

I should note that video is working in both mpv and vlc here - on plasma5 / mga 8 / intel gfx.

I tried other video players. These are working fine: MPlayer, Parole, Xine.

KMPlayer, Totem and VLC don't work and SMP only partially as described above.

I just like to add that when I wrote that certain video players didn't work this meant that their screen was black (or blue for the KMPlayer), but the sound was fine.

I don't know whether I did it correctly. I tried "mpv --vo=[driver]" and got this result:
Option vo: filter name expected.
Error parsing option vo (option parameter could not be parsed)
Setting commandline option --vo=[driver] failed.

Exiting... (Fatal error)

I just discovered something amazing. When I use the desktop environment "GNOME (Wayland)" all video players work properly. But with the normal Gnome desktop (as well as Cinnamon and Plasma) there are the mentioned problems with KMPlayer, Totem, VLC and SMPlayer.

Under Gnome Wayland the Chromium Browser which showed an empty window beforehand works again properly.

I am wondering. If Wayland helps how can I start Plasma with Wayland?

(In reply to Béat E from comment #9)
> Under Gnome Wayland the Chromium Browser which showed an empty window
> beforehand works again properly.
> 
> I am wondering. If Wayland helps how can I start Plasma with Wayland?

See https://github.com/shlomif/shlomif-computer-settings/blob/master/shlomif-settings/home-bin-executables/bin/plasmawayland.sh .

"[driver]" is a placeholder and needs to be replaced "--vo=drm", "--vo=xcb" / etc,

I tried "--vo=drm", "--vo=xcb" ass placeholders and this didn't work. How do I find my driver for this command?

I am using Plasma Wayland now and there are none of the mentioned problems (but others (bug #26543).

I haven't got any answer on comment 11. 
I tried "mpv --vo=drm", "mpv --vo=xcb" and this didn't work. What do I need to do in order to find the right placeholder?

I don't know what happened. But all the problems are gone now. I have no idea why. I can use all the video players without difficulty. This bug can be closed.

In the meantime I found out what fixed the problem. It was the update of the X11 packages.

Thanks for the info that the X11 update fixed the problem. Closing the bug
report as fixed.

Resolution: (none) => FIXED
Status: NEW => RESOLVED
CC: (none) => davidwhodgins