I am having two issues, they are probably related to each other.
1. In each of my portfolios i cannot choose a portfolio parent.
2. In my page which i called Portfolio, i have selected the template ‘Portfolio – Portfolio List Page’ but when i load the page the page is blank (as in the navigations is there and it works like a page but the main section is blank).
I think it had to do with my Portfolio page having the homepage as the parent. So i made the Portfolio page have no parent and then I was able to give each Portfolio item the parent of Portfolio.
I was not getting a 404 error though, so you may have a different issue.