I have had this problem for a couple of years, probably due to the fact that all my settings get transferred when I log into Chrome: Most videos on the net play fine using Chrome, but on certain site none of the videos will play. This is not due to wrong codecs or something, because opening the same video using Chrome's Private Browsing Mode will play it just fine.

Since most (all?) extensions are disabled when using Private Browsing Mode, I guessed the problem had to be found in my extensions, so I disabled all and also disabled developer mode. The problem persisted ...

• Example html5 video from the W3C plays fine
• This video from Pecha Kucha does not