Probably something wrong with the video. The original doesn't play on my Mac (with VP8/VP9 support) either.

VLC reports the following warnings and errors:

avcodec warning: plane 0: pitch not aligned (1056%64): disabling direct rendering
mkv warning: find KaxCues FIXME
mkv debug: Unknown (N11libmatroska11KaxSeekHeadE)
mkv debug: MKV/Ebml Parser: m_el[mi_level] == NULL
mkv warning: EOF
mkv warning: cannot get block EOF?
main debug: EOF reached

So the block with file offsets for all the seeks points in the file are likely incorrect.

Ah, another wondershare video btw.. not the first time that product has been creating problems.

In T306778#7876803, @TheDJ wrote:

Ah, another wondershare video btw.. not the first time that product has been creating problems.

Oh, that's good to know (sad though). Thanks for the investigation and all the pointers, @TheDJ!

I remuxed the video, which seems to have solved the problem. ffmpeg -i input.webm -c copy output.webm

In T306778#7887012, @TheDJ wrote:

I remuxed the video, which seems to have solved the problem. ffmpeg -i input.webm -c copy output.webm

@TheDJ Would you mind sharing how you fixed the issue? I have a few more videos that I made with Wondershare and would like to apply the fix myself. Thanks a lot!

@TheDJ Thanks to @awight, I was able to fix the issue on the other videos too.

In T306778#7934053, @JStrodt_WMDE wrote:

@TheDJ Thanks to @awight, I was able to fix the issue on the other videos too.

tl;dr, we used your ffmpeg commandline, thank you!