Third-party video providers can be unreliable because the servers sometimes block Hubs due to high traffic (https://hubs.mozilla.com/docs/hubs-faq.html#can-i-play-video-content-in-hubs). For important videos it may be helpful to upload them directly to spoke (if they are small enough), or self-host them on your own web hosting site. For dropbox you may need to change the url to use ?raw=1 instead of ?dl=0 please note that dropbox may also block Hubs servers temporarily if there are too many calls being made to their servers simultaneously, so it might work for small events, or events where people come by a few a time over a number of hours; if you are expecting a large number of participants at once you may wish to consider other hosting options.