I have been trying to address this issue for a whole day and this worked for me:
When DNS was set up on my domain, I wass forced to choose a primary domain between www . domain . com and domain . com. I chose www . domain . com and cPanel aliased domain .com to it.
However, when I set up AbanteCart, in Admin > System > Store Details > Store URL, I used domain . com. When I changed it to www . domain . com everything worked as it should.
So, make sure the URL for the store is the same as the domain used in the setup of the control panel.