Hi HLX,
this is not JDownloaders' fault:
This video can be played in Chrome but fails in Firefox and also when you're [manually] downloading it via browser (tested with: VLC Media Player, Windows Media Player).
Direct URL to the vide for testing:
Code:
video.twimg.com/amplify_video/1704654962834214912/vid/avc1/1280x720/50-birosgYs_rDGX.mp4?tag=14
Twitter is using a so called "iso5 header marker" for this video which is unsupported by many video players.
ffmpeg can remux it
Example command:
ffmpeg -i NP5ZADHZ6Ss29OFh.mp4 -c copy test.mp4
Possible workaround:
1. Stop all downloads in JD and do Rightclick on that video -> More -> Reset
2. Go to Settings -> Plugins -> twitter.com -> Enable setting "Videos: Prefer HLS over http download?"
3. Now try downloading the video again. If you are lucky, the video will be downloaded with another internal container format now.
EDIT
I just wanted to confirm that this workaround is working fine for your particular video.