@Andrea There are a lot of posts on this thread, so I apologize if I've misread something. If I am reading the dumps correctly, it appears that you do not have enough memory for 3.0. Please see the minimum requirements here to confirm.
Andrea, to your original message - http://dev.zenoss.org/trac/ticket/7033. I believe that this was an error in the installation process instructions not being followed completely. Hopefully this issue has not reproduced for you.