Upgrade not working
I am having the same problem. I uninstalled the previous version running in cs3 on mac 10.4.11 then installed 4.5.1, made a change to the cart ,now the page does not load. I stripped out all the code relating to ecart and the page loads. I proceeded to add a add to cart button and the page does not load.
So I know this does not help you, but at least you know your not alone. I too am looking for answers if anyone has any ideas.