Comment 6 for bug 1670410

I just had this same issue occur with an upgrade from trusty to xenial.

Wouldn't even let me uninstall the squid package from the newly upgraded system until I created the directory '/var/spool/squid3', then the package worked and I could uninstall/reinstall.

Luckily it was in a virtual machine, so I was able to revert to the pre-upgrade state to see what was going on before the upgrade and verified that the /var/spool/squid3 directory existed before the upgrade started so it's been removed by *something* at some point before the attempt to uninstall squid.