Recently, some customers have complained that they are unable to checkout after making an order, but I thought it was a couple of isolated incidents. Yesterday, one customer, who has made purchases before, said he had tried placing his order both at home and at work and neither would let him checkout. I looked at the customers and it does show his last login attempt was yesterday. Unfortunately, he and the other customers never give any specific details about their problems, so I don't know how far into the checkout process anyone can get.

I already saw a similar thread about how someone was trying to use free shipping but the module wasn't on, but it doesn't apply to this situation since I only use flat rate shipping for all items.

I'm using Zen Cart 1.3.7. Zen Cart 1.3.7, Database Patch Level: 1.3.7, and PHP Version 5.2.3.

I tried creating a customer, and the whole checkout process works fine for me.

If anyone can help me figure this out, I'd really appreciate it. Let me know if I need to provide more details.