@Wayne-Workman @Tom-Elliott Solved the problem. Somehow curl had a problem with me being behind a proxy, no idea why though, never had this problem before. As a result, bzimage, bzimage32, init.xz and init_32.xz were not downloaded correctly. I solved this problem by using wget to manually download those files (the download links are in /lib/common/functions.sh).
Also another problem in /lib/common/functions.sh: I was not able to add the repository for php5-5.6, so I had to comment out those lines. In my case this is not a problem, as Iām now using Ubuntu 15.04, which already has php 5.6 preinstalled (at least if you dist-upgrade and update).
Thanks again for the very fast help. š