Based on what you show in your log extract, you are trying to install much more than "squid" isn't it?
Furthermore, you don't show command used to launch installation.
If you don't want, for some reason (that could be discussed further BTW) to install using Zentyal ISO, based on Ubuntu 12.04, I would suggest that you first edit sources.list in order to include Zentyal repository then install Zentyal minimum components before moving ahead using Zentyal GUI.
I would say that minimum package list is something like:
zentyal-common
zentyal-core
zentyal-software
(I never tried such install with V3.0 but would be surprised if this doesn't work smoothly)
Other Zentyal components will be automatically added due to dependencies.
Once done, connect with your browser
Better do this from basic 12.04 Ubuntu server installation. No need to install Apache, Squid or whatever else before installing Zentyal.