Thank you all for the replies!
Today we tried to fix it ourselves and could make it to the next KTM-dealer (Oregon Motocycle Adventures ! ) and found out that the slave cylinder had a crack inside that was hard to see. They gave me the hole thing from their demo bike, likewise the chainguard that was gone. We put on new chain and sprockets, new oil and now I hope everything is fine. Was not cheap...
The braker, is it the thing that is above the front sprocket? I have got a metal part there and have already wondered why there is a screw missing (into zhe engine). Could it be that this remains from an original part that had been replaced?