do-release-upgrade hangs at downloading openjdk-6-jre-headless

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http://askubuntu.com – I'm trying to do do-release-upgrade (also tried the graphical version before - same issue with less details). It always get stuck downloading openjdk-6-jre-headless. The exact line 3% [18 openjdk-6-jre-headless 27.1 MB/27.3 MB 99%] It always exactly downloads 27.1 MB, then stops and then it tries again. I already changed the server but no difference (tried http://de.archive.ubuntu.com/ubuntu/ and http://en.archive.ubuntu.com/ubuntu/ so far). The direct link to the file is this. This file is about 26 MB (again different?). Why is this? What's the problem? How can I fix it? Or work around (HowTos)