tannenzaepfle: Based upon your description, I find I am having a similar problem (if not the same).
I can download troll trouble, but after verifying it kills the data and starts downloading all over again, in an infinite cycle.
It is not possible for me to influence this process. Everytime, as soon as the download is finished, the Launcher starts to overwrite the .dlc file.
Killing the launcher after it completes also kills the verification process, and as soon as thats finished the download starts again... you get the idea.
No solution on my side so far...
I had the cycle problem too (always restart-download), then I switched off firewall (I use avast). The this download-cycle problem ceased to occur but intstead the above described similar problem comes (which also seems to have the same result but with a little different outlook).