Many videos on Wikipedia are no longer working for me. I've tested on my Debian Jessie system with Chromium 31.0.1650.63 and Iceweasel 24.2.0, and on my Ubuntu 13.10 system with Firefox 26.
When I click to play, I get an overlay box with no content (see attachment).
Videos that don't work:
https://en.wikipedia.org/wiki/Wikipedia:Training/For_students/Text_editing
https://outreach.wikimedia.org/wiki/Education_Portal/Tips_and_Resources#Videos
Videos that work:
The video here in mainspace: https://en.wikipedia.org/wiki/Beer_bottle#Closure
All videos when a page is in preview mode.
(I guess that some necessary javascript isn't being loaded in the cases of videos that don't work?)
Version: unspecified
Severity: critical