Customers bumped to checkout step 2 after submitting order

So I have done multiple test orders, all of which worked fine through my macs and iPad, on different browsers. I just launched my store and had a customer tell me that after submitting the order in PayPal they get returned to checkout step 2 rather than checkout success page? I'm using paypal express checkout because I read it is more reliable even though I have the express buttons turned off. Any idea why this would happen? They were using a PC with Firefox? Maybe I have a bad setting? I'd hate to be losing orders and not even know about it. Any help would be great!

Where was this customer ? Do you have any zones set up ?

Search the forums for "step 2" return for some threads that address this problem.

I don't have any payment zones, I just have one tax zone setup for California.

I have done a lot of searching, most of the posts that I found have been closed without any real solutions. I have the latest version of zen cart, 1.39h, which is what the site was built on. The only contribution applied to it is recaptcha. Paypal Express Checkout is the only payment gateway I have setup. I originally had Paypal Standard but then read it was better to use Express as it was more reliable. I have it set to Not use Express checkout however, so it takes you through the whole checkout process.

I do have AutoReturn set to on in my Paypal account, I read one thread where somebody said it worked for them to turn it off? I sell downloadable items so I do need customers to hit the checkout success screen though.

Is there anything else I should be looking at? I have not been able to replicate this issue and have done multiple checkouts, all have been fine.

But I have now had 4 customers complain about this in the first day the site was live :-( and I can't expect my customers to troubleshoot for me.

I turned off Auto Return in my paypal account, and sent a message to my customers to try their purchase again, and one has done so and was successful. So did this fix the problem or did it just happen to work right this time?