I'm sorry, I don't have the log anymore (and we switched to 13.2 PXE in the meantime, which is working just fine). Anyhow, it looked like the content of http://download.opensuse.org/factory/repo/oss/boot/x86_64/loader/ was just incomplete: the initrd was just 1,8MB big, and there were multiple files with 0 size. This seems to have been fixed in the meantime, now the file sizes look more correct. I suppose this can be closed, then.