A couple of days ago I started to get these messages when trying to watch videos on Youtube and a couple of other sites. (Using Chromium browser) Normally starts by just seeing a black screen with Missing Plug-in faintly visible, but will also ask me to give permission to run both at top (yellow bar), and if I tell it to 'allow' it follows up with the white puzzle piece on black background requesting permission. Occasionally I have seen the message telling me the flash-player has crashed, too. Giving permission does not fix the problem.

Have also navigated to 'about:plugins' and checked that Adobe flash is enabled and permitted.

I have just done a fresh installation of Lubuntu 11.10 as a last resort but problem still not fixed.