In the developer tools console there is a mixed content warning:

  Mixed Content: The page at 'https://media.ccc.de/v/32c3-7435-replication_prohibited#video' was loaded over HTTPS, but requested an insecure video 'http://ftp.halifax.rwth-aachen.de/ccc/congress/2015/h264-hd-web/32c3-7435-en-Replication_Prohibited.mp4'. This content should also be served over HTTPS.

FWIW Firefox plays it just fine