windows - Why have embedded YouTube videos stopped working in Google Chrome?

Yesterday I could watch videos at the Khan Academy site, today they don't work. Instead of the video I get a black screen with a link to "Get Adobe Flash player". Clicking the link takes me to a page telling me "Your Google Chrome browser already includes Adobe Flash Player built-in. Google Chrome will automatically update when new versions of Flash Player are available." All sites with embedded YouTube videos have this problem.

Here's an example of a video that doesn't work: https://www.khanacademy.org/video/unit-circle-definition-of-trig-functions-1

Here's what I see instead:

And the Adobe Flash page:

The above video plays fine in Firefox, as do all other embedded YouTube videos.

Watching videos at youtube.com still works.

I have cleared everything since the beginning of time with Tools->Clear browsing data.

System information:

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Chrome Version (type about:version into your omnibox):
Google Chrome   31.0.1650.57 (Official Build 235101) m
OS      Windows
Blink   537.36 (@161986)
JavaScript      V8 3.21.18.7
Flash   11.9.900.152

Operating System (Windows 7/8/Vista/XP, Mac, Linux, Android, iOS):
Windows 7 Ultimate 64bit

Extensions (type Chrome:extensions into your omnibox):
Adblock Plus 1.6.1
Antisocial 0.2.4
DoNotTrackMe: Online Privacy Protection 3.1.1021
Download Master 4.0.0.2
Easy Auto Refresh 2.9
FVD Video Downloader 5.5.8
Google Dictionary (by Google) 3.0.19
Google Docs 0.5
Highlight to Search 1.0.37
HTTPS Everywhere 2013.10.16
LastPass 3.0.14
New Tab Redirect! 2.1.1
Personal Blocklist (by Google) 2.4.1
Pinboard 2.2.1
RSS Subscription Extension (by Google) 2.2.2
Search by Image (by Google) 1.5.0
Vanilla Cookie Manager 1.3.2

What has gone wrong and how can it be fixed?

Answer

The FVD Video Downloader extension was the cause of the problem. Disabling it fixed the problem.

Thank you to the commenters for steering me in the right direction.