Customers unable to complete orders

I’ve had multiple (4) customers complain to me that when they were attempting to check out with items in their shopping cart, they get kicked back to the login page. They login again and then get kicked back at the same point. They’re not able to complete their transaction. The last customer said it happened when she entered her CC expiration date.

Aside from upgrading to the now subscription-based cart, how can I fix this issue?

Thanks

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