Add support for producing videos compatible with Twitter

Which videos have you tried? Did you try mp4, webm, gif and see, if they work? Have you tried converting them using some kind of videoconverter-tool, which is known to be able to produce videos accepted by Twitter?

I believe twitter likes MP4/H264 encoding, which REAPER can output via ffpmeg or (I think) VLC.

So the fix would be to use FFMPEG instead of VLC?

Now it works, using MP4/H264 encoding. In the meantime I installed FFmpeg, so I guess that is what made the difference to Twitter.