Comment 10 for bug 778638

To re-enable network access, you can add the following to /etc/appamor.d/usr.bin.evince to the '/usr/bin/evince-thumbnailer' stanza:
  # TCP/UDP network access
  network inet stream,
  network inet6 stream,
  network inet dgram,
  network inet6 dgram,

Then do:
$ sudo apparmor_parser -r /etc/apparmor.d/usr.bin.evince

It should be noted that the default install of Ubuntu uses 'Local Files Only' for thumbnailing via nautilus, and changes this preference back to the default should also workaround this issue.