I have a site with eShop and a few products that has been working fine for a while, but all of a sudden about 4-5 days ago people started getting 404 errors during checkout, after filling out the form entirely and submitting it. The only payment method is Paypal. Nothing on the site was changed when this starting happening, it just sort of came out of the blue.
The url when you get to the 404 page is still mysite.com/shopping-cart/checkout/, which is the correct url, and if you submit the form when it's blank it will work correctly and come up with the 'please fill in these fields' message at the top (but of course as soon as you do fill them out you get the 404).
I tried updating both WordPress and eShop to the latest version (both were 1 version behind when I first noticed the issue), tried deactivating/reactivating it, hasn't fixed it. I haven't seen anyone else with this problem, either.