I've had 1.5.1 setup stable running for many years. I only use paypal pro merchant account it has worked flawlessly for many years.
Suddenly in the last week or so checkout for the users has become very glitchy.

1) Orders submitted multiple times - I figure based on the time stamps that this is the result of the customer hitting the submit button multiple times because they are not getting a checkout confirmation. But that's just a guess.

2) Not all orders come through. I see them in the PayPal admin, but they do not appear in the ZenCart new orders. Sometimes those are duplicates/triplicates too none of which appear in the new orders admin.

3) But some orders come through just fine.

One customer reported she was charged twice (which she was) but never got a checkout page or email confirmation, and when she logs into her account the items she ordered are still in her cart.

Anyone got an ideas how to hunt down or fix this glitch? Seems like some breakdown in the checkout process that doesn't push the order into the db or give the customer the confirmation page. No confirm email goes out on the orders with these errors.

Aside: I was going to upgrade versions and install a new template, but the current host doesn't offer PHP 7.x so as I tried to install fresh 1.5.7 in a new directory (following the directions, because I wanted to keep the database tables, customers etc) it would not install. I'll may to switch hosts or hosting plans, and will do that at some point.