Download

There are lots of reports of people having trouble with downloads
One frequent culprit is Express files with many hundreds of assets; in that case, the best workaround is to split the video timeline across multiple pages, download them individually, then merge them together.

Alternatively, consider:
1- reloading the page (F5)
2- Clearing the cache
3- Try another browser (compatible browsers are Safari, Edge, Firefox and Chrome)