Well both videos work for me in Safari, Opera Developer and Epic which are the only browsers I use that I allow Flash on as they are not my everyday general use browsers.
Digging round in Develop mode in Safari and errors - I get hidden away error > [Warning] [blocked] The page at about:blank was not allowed to display insecure content from http://www.youtube.com/embed/IDENTIFIER.
'This with the VTR forum page being https trying to show youtube vids edited to be http (from https)
Then when I reload the VTR page as http the vids work - which points at a mixed mode error with https and http.
Same behaviour in Safari 10.0.2, Firefox 50.1.0, and Chrome 55.0.2883.95
So it seems while viewing the VTR site in https secure mode it will block any embedded 'insecure' http content. As forum's plus-in / extension for showing embeded https video doesn't function - hence the old hack having to edit the 's' out to get it play nice even in http mode.
And this odd behaviour I now recall started when the site was made https.
Argh I see the issue is with the mixed content. I don't think things have been setup quite right on the https side of things and certificate. External content should show and play but as we have no control over external content it should show up as mixed content ie non https or it could even be the browser itself blocking mixed content over https.