webm-Video can't play as guest

I just converted and uploaded a few videos of my last nightout. I converted them into webm-format. Me as the uploader can watch them directly in my webbrowser.
The shared link does not work. It always want to download, even on the same machine and the same browser one tab away.
I always get the message “The video could not be loaded either because the server…” WHATS that?

Logged in, same user or shared user, I can play that video in the browser.