I noticed that when we tried to add “outside” URL: for our stream in event . Then we tried to start it… there were not any previewplayer on stream (picture)
By “outside URL” you mean YouTube URL or a third-party RTMP stream URL?
Yes, there should be a video player there. The preview player is identical to the event player (that’s kind-of the point: it being identical reflects exactly the live event page) but it loads differently so in some cases it needs a manual refresh for the whole page before it starts playind. Did you try to restart the page?
With third-party RTMP streams usually the problem is in CORS settings, but if you allow either '.slipmat.io' or '*' everything should work.