This one had me scratching my head for a while…
Built a new Zen Cart site, registered as a customer and added some products to my cart. All fine until I try to checkout, when I just get redirected to the home page after re-entering my credentials. Occasionally I’d get the ‘whoops, session expired’ message, but mostly KC was fairly unhelpful in its responses.
Turns out you need to go to ADMIN-CONFIG-SESSIONS and set the ‘Recreate Session’ flag to ‘false’. Absolutely no idea why, but it seems to fix the problem.
Gotta love it.