Have you tried switching your browser? Something happened to me so I switched over to Firefox for a few months before going back to Chrome. Might help.
Using a different browser didn't work. I'm only having this problem on the forum. Any ideas-suggestions?
Any time things go wrong with computers I search for registry issues with ccleaner, scan for spies with superantispyware, and if neither do the trick I run malwarebytes.
By that time the problem is gone.
Tried them all (except I use Avast) and it didn't help.
Hmm. Ya man sounds like a case of old hardware. YouTube (and numerous other) sites keep evolving in their coding and it places more demands on the hardware. Could it be software? Maybe but I think web browsing/tabs with YouTube take a toll on old hardware. What OS are you running? 7 is starting to be phased out lately.
I use avast too. ....tho it's getting pretty naggy.
It is only on the forum? I am not really certain why. Did you do anything to break or disable Flash? I think if you visit YouTube's actual site now it will try to serve video as HTML5 rather than Flash, but when embedding it still used Flash. You may wish to try visiting https://get.adobe.com/flashplayer/ and reinstalling Flash.
Otherwise, I have few ideas. There are ways to set up Firefox so that content only loads when you click on it, but that should mean that if you click on the black square that the video will load.