Thank you very much syncrow!
I can see the bagslots now and there is no more error. BUT the item quality color glow/border around the icons is not showing and when I hoover a bagslot I get a green texture (like a missing texture normaly it shoud display "glowTex.tga" I guess its something wrong with media.lua).
How it shoud look:
http://cdn-wow.mmoui.com/preview/pvw61654.jpg
How it looks:
http://i.imgur.com/WUZjIRn.jpg