I have unistalled Opera and Vivaldi and will stick to Google Chrome, FF and Palemoon.

Thanks anyways.

One weird thing you can try though is to create a new user in windows, launch Opera with that user and see if there's any difference. There normally shouldn't be any difference, but maybe there's something funky with your windows user profile (even if the computer is newly set up). If it makes a difference, you can investigate from there.

Video Decode: Software only, hardware acceleration unavailable

That indeed might be related. If you didn't already, go into power options in windows and edit the advanced settings for your current power profile (or create a new one or modify a different one etc.). Maybe tweak the video settings to optimize for full video quality. You might also try looking in the ati settings to make sure settings are tweaked so the card is running at full capability/performance. Maybe the card is running is some power saving mode or something weird. You might want to make sure your monitor is running at 60Hz instead of 50. Stuff like that. Anything that might get Opera to enable hardware acceleration. Might have to restart after making the system changes. (I doubt this is the issue, but something to try.)

No, Vivaldi can't play it either.

I've already imagined that. It's because Vivaldi, like Opera, doesn't have its own h264 video decoder so they depend o the OS to play those videos but somehow, even with the OS being able to play the video natively, they can't play it.
Do you have 32 or 64 bits Opera?

I use Opera 43.0.2442.1144 (PGO) 64-bit version and Vivaldi 1.7.735.46 (Stable channel) (32-Bit)

No, Vivaldi can't play it either.

I've already imagined that. It's because Vivaldi, like Opera, doesn't have its own h264 video decoder so they depend o the OS to play those videos but somehow, even with the OS being able to play the video natively, they can't play it.

Do you have 32 or 64 bits Opera?

OK, I did now enable it, cleaned all history data with CTRL + SHIFT + DEL and restarted Opera.
Then played the video again.
No difference.
The video is still black, only the sound works

Did you try cleaning the cache and media cache?

Yes, FireFox, Google Chrome and Palemoon (all without the Flash Player) can play the video fine.

Did you check with Vivaldi or any other Chromium based browser that not Chrome?

I've cleaned the cache with this key combination, and additional also cleaned it with CCleaner. Opera is there under applications.

I will try Vivaldi and see how it goes.

OK, I did now enable it, cleaned all history data with CTRL + SHIFT + DEL and restarted Opera.
Then played the video again.
No difference.
The video is still black, only the sound works

Did you try cleaning the cache and media cache?

Yes, FireFox, Google Chrome and Palemoon (all without the Flash Player) can play the video fine.

Did you check with Vivaldi or any other Chromium based browser that not Chrome?

Hmm, this is very strange.
You said in your OP that Firefox plays the video fine, without Flash installed.
From my tests, this seems to indicate that Firefox is playing the video using HTML5 if Flash is not installed, although it appears to use Flash as the first option if it is installed.
From your tests, it does appear as if the HTML5 player is working correctly in your Opera installation, but it won't play that particular video correctly, even though it has presumably defaulted to it as the Flash player isn't available.
Could you install the PPAPI Flash plugin temporarily just to see if Opera does then play the video?
You can download it here and you can always immediately uninstall it after the test if you don't want to keep Flash on your system!

Now with Flash installed, it works!
But i don't wan't the Flash-Player on my computer, so i will uninstall it.

I hope Opera programmers can find the solution and fix this HTML5 problem.

Yes, FireFox, Google Chrome and Palemoon (all without the Flash Player) can play the video fine.

Opera plays it but there is only the sound and the video stays black.
IE cant play the video at all, it shows just an error message.

Video Decode: Software only, hardware acceleration unavailable

• Video Encode: Hardware accelerated

This may be a problem when running h264 videos. See if enabling opera://flags/#ignore-gpu-blacklist makes any difference.

OK, I did now enable it, cleaned all history data with CTRL + SHIFT + DEL and restarted Opera.
Then played the video again.
No difference.
The video is still black, only the sound works.

Video Decode: Software only, hardware acceleration unavailable

• Video Encode: Hardware accelerated

This may be a problem when running h264 videos. See if enabling opera://flags/#ignore-gpu-blacklist makes any difference.

Go here and see what formats your browsers support.
It's a bit out of date now, but should show if the browser supports HTML5 video.
It's possible that Opera is trying to play the video using HTML5, but is failing and not dropping back to the Flash player.

Opera plays all 3 videos fine, all with sound and video.

IE can only play the H.264/MP4 (with sound and video).

Did you check the test page I linked to in an earlier post?
Does the first video (H.264/MP4) play OK in Opera (and IE for that matter)?

Opera plays all 3 videos fine, all with sound and video.

IE can only play the H.264/MP4 (with sound and video).

All other browser don't have Flash either. But they can play this video.

Check opera://gpu/ to see if there are any error message under 'log messages'. Also check what it says about video decode/encode.

Video Decode: Software only, hardware acceleration unavailable

• Video Encode: Hardware accelerated

Log Messages
GpuProcessHostUIShim: The GPU process exited normally. Everything is okay.

All other browser don't have Flash either. But they can play this video.

Check opera://gpu/ to see if there are any error message under 'log messages'. Also check what it says about video decode/encode.

but it works in Media Player....
I don't have any codec pack installed but the video works fine.
BTW, K-lite or any other may help.

If the video plays on WMP even without any codec have been installed, it means that Windows is able to play it natively so installing a codec pack wouldn't make any difference.

I forgot Flash it's not installed on the system.

Do you use any adblocker into FF or Chrome?

Some times this happens to me in some sites because the AdbBocker blocks part of ads but there is an Overlay hidden the video.

Another option, check site options with Site Settings and / or in Adobe Flash and PC options for "Fail.to"

https://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager07.html

https://forums.adobe.com/thread/977699

The most annoying:

With Opera i have at least sound, but my IE 11.0.9600.18537 can't play the video at all (no sound, no video).

but it works in Media Player....
I don't have any codec pack installed but the video works fine.
BTW, K-lite or any other may help.

Installed now K-Lite Full Codec Package and rebootet.

But it didn't help either.

Still same problem.