Quicktime Plugin not working in any browser

Hi,

I recently bought the new MacBook, I used a 2014 MacBook Pro before that.

Sometimes I use a kind of online course for studying, which consists of videos (in Quicktime format) and flash cards. On my MacBook Pro, I always was able to play these videos - with Firefox.

Now, on the new MacBook, I just can't get them to play. I've tried Safari, Chrome, Firefox, Opera, installed Flash Player and Java, downloaded Quick Time Player 7 - it's always the same.

When the clip should start in Safari, it says "Missing Plugin", "On this website is a content that needs Qucktime Player" -> "More Infos" -> Apple Website "

QuickTime is built into Mac OS X."

Well, I use Mac OS X (10.11).

I've also already tried formatting my hard drive and re-installing OS X - didn't change anything. I don't know what else I can do...

Thanks