@burnout426 I have already checked several times (and ways): all this N-version packages are not supported by my version of Windows 10
Here is full log:
https://codeda.com/data/media-internals.txt
If you have some better mp4 for testing I will test on it
Also tried all 5 variants in "Choose ANGLE graphics backend" and hardware acceleration in opera://settings/system and opera://flags/#disable-accelerated-video-decode and opera://flags/#disable-accelerated-video-encode

@burnout426 it has happened after upgrading Windows 7 to Windows 10 with Media Creation Tool
I have created new Windows account, installed and tested on all 4 versions just after installation and had same problem on every of them. Didn't try Vivaldi there because it was first time I ever installed it.
And yes, it happens on video-only mp4 files. But if I create audio-only mp4 it plays fine
Also tried all fixes from Codec Tweak Tool > General > Fixes (including Reset Media Foundation). Even after system reboot nothing changes for the problem.

@burnout426 already tried both dism and sfc. All system files are OK. It was my first idea.
And of course installing Windows from scratch will help. But I have no option to do it now.

Found Legacy Components in optionalfeatures.exe. It's not checked and child "Direct Play" also not checked.
I've enabled it and rebooted - no changes, disabled and rebooted - still no changes.

@burnout426 said in [Duplicated]ffmpeg video decoder problem:

Direct Play

What is it? How can check if it enabled?

I've removed WMP, check that it was actually removed after rebooting, then added the component back and rebooted again. WMP still plays h264 and Opera still can't.

Also, as I remember, the only thing that I have been asked to remove to continue updating Windows 7 to Windows 10 was Windows Media Player. But after update it was already installed.

Is there is a way to find what dlls (or other system files) my Opera tries to use for h264 decoding?

Found Legacy Components in optionalfeatures.exe. It's not checked and child "Direct Play" also not checked.
I've enabled it and rebooted - no changes, disabled and rebooted - still no changes.

@burnout426 said in [Duplicated]ffmpeg video decoder problem:

Direct Play

What is it? How can check if it enabled?

I've removed WMP, check that it was actually removed after rebooting, then added the component back and rebooted again. WMP still plays h264 and Opera still can't.

Also, as I remember, the only thing that I have been asked to remove to continue updating Windows 7 to Windows 10 was Windows Media Player. But after update it was already installed.

@burnout426 only ESET NOD32 Antivirus. I've just removed it (and its services) from windows startup and re-tested. The problem is still present.

What is a difference between decoding h264 in Chrome/Edge (where is no issue) and Opera/Vivaldi?

@burnout426 already tried both dism and sfc. All system files are OK. It was my first idea.
And of course installing Windows from scratch will help. But I have no option to do it now.

@burnout426 it has happened after upgrading Windows 7 to Windows 10 with Media Creation Tool
I have created new Windows account, installed and tested on all 4 versions just after installation and had same problem on every of them. Didn't try Vivaldi there because it was first time I ever installed it.
And yes, it happens on video-only mp4 files. But if I create audio-only mp4 it plays fine
Also tried all fixes from Codec Tweak Tool > General > Fixes (including Reset Media Foundation). Even after system reboot nothing changes for the problem.

@burnout426 I have already checked several times (and ways): all this N-version packages are not supported by my version of Windows 10
Here is full log:
https://codeda.com/data/media-internals.txt
If you have some better mp4 for testing I will test on it
Also tried all 5 variants in "Choose ANGLE graphics backend" and hardware acceleration in opera://settings/system and opera://flags/#disable-accelerated-video-decode and opera://flags/#disable-accelerated-video-encode

@leocg downloaded mp4 plays in Windows Media Player but Vivaldi 5.6 has same problem as my Opera.

@leocg thanks.
I have this problem in my Opera on every mp4/h264 video so far. And no problems with webm/vp8. For ex. if I convert the video to webm (https://codeda.com/data/syncTest.webm) I can play it in my Opera.
YouTube source is also OK https://www.youtube.com/watch?v=ucZl6vQ_8Uo