No answers, but some possible trouble-shooting questions that might move us ahead.
You do have libflashplayer.so in the plugins directory. That's the name of the official Adobe flash player plugin.
CD to the plugins directory and run ls -l and check the permissions. Not that I suspect that to reveal much, but there's no such thing as too little information.
Also, in Iceweasel, go to Tools-->Addons-->Plugins and verify that it's showing a flash plugin as installed.
If you have another browser, such as Epiphany or Sea Monkey, you might also test a YouTube in that to see what happens.