Bug#567437: solved this/my problem by unmounting btrfs and xfs filesystems

[JPH]

I ran into this very problem today and Googling around I realised I had
been busy with XFS and BTRFS filesystems today. After unmounting (with
-l for lazy as btrfs hung) these filesystems, apt-get started to work
properly again. It even rebooted properly (always interesting when grub 
starts to complain ..)

Hope this helps,

JP