Hi Flixxel!
Thanks for taking the time!
Unfortunately this doesn’t solve my problem. In your code you’re using another swf
to import assets from. Yours was exported using Flash CS5.5 as an AVM2 movie.
Problem is, I’m not using Flash CSx to create my assets and my application just can
export up to swf8.
I stumbled across this thread: Link
The user seems to have the same problem and it could be solved
by forcing type “swflite” on all targets as @singmajesty pointed out.
I just don’t know what this means. 
I tried:
<library path="assets/lib.swf" generate="true" preload="true" target="swflite" />
but this didn’t change anything either.