The syslog states nothing unusual and there aren’t any error logs present =/
I’m kinda out of ideas right now, so I’d gladly take any advice …
Edit: Found the solution !!
As my discourse instance isn’t ssl secured yet, I embedded non-secure content on a secure website. This was blocked in my browser (firefox) because of security issues ! In e.g. interntet explorer everything works fine 
thanks a lot for the help, @bartv and @Mittineague !
ISSUE SOLVED