I know this question is a little old, but have come across this twice now. Once with puppet, once with virtualbox. It cropped up again and I found something that worked. It's a variation on serverfault.com.

Instead of relying on dpkg-statoverride --remove /path/to/offending/file.ext which, throws the error

dpkg: unrecoverable fatal error, aborting:
syntax error: unknown group offendinggroup in statoverride file
You can get fancy with sed, or you can simply open /var/lib/dpkg/statoverride in your favorite text editor and remove only the lines containing the item(s) that were causing the error. I found that this is a documented bug, for both Debian, and Ubuntu,

https://bugs.launchpad.net/ubuntu/+source/dpkg/+bug/593615
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=705752
That said, I just fixed mine by manually editing out any entry that threw an error, and I am happily downloading packages again. Hope this helps the next person, who visits this number 1 Google search result for this problem.