After that happens, what's in their cart at that stage?
Do you know whether the same issue occurs if someone had something in their cart from a previous shopping session, but had logged out and never completed checkout with the items in their cart? Does it display the shopping_cart page again with a note that their cart contents have been merged? Or does the "whoops" message display before that can happen?
Zen Cart doesn't use "time_out.php" unless perhaps you're using some addon that rewrites URLs. Do you have such an addon installed?
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