On your video page, try manually restarting the transcoding. There are two options on the three dots. A lot of my videos fail during the first attempt but then work just fine when I start one or the other transcoding manually. With longer videos, the Web Video version tends to work better.

That’s someone filming a monitor. The camera is fixed and doesn’t follow the people.

For Linux there’s puddletag, which is very similar to mp3tag