I know this is an older post, but I had a similar issue with Shopify once, and maybe my experience can help. It could be a problem with how the page is rendering or an issue with the theme or app integration. I worked with a client whose Shopify store showed a blank page after clicking the buy button, and it turned out to be a JavaScript conflict with an app they were using.
One thing you could try is clearing your cache or disabling apps one by one to see if that helps. Also, if you're ever thinking about improving site performance, I recently worked with
Refined Commerce, and their Hyvä theme for Magento 2 really boosted speed and avoided such issues.