Same issue for me, yesterday it was even worse, for almost the whole day it knocked sales down to zero. This has actually been an off and on problem for a while as far as I can see. Some things I noticed that might be useful if we all collaborate, as it seems Shopify continues to deny there is a problem and support is useless so far:
Checkouts work fine for me so I can't replicate the error myself.
My store uses the 1 page checkout
I have noticed that when the checkout error is happening, people using Apple Pay seem to be able to make purchases. Maybe just a coincidence but worth noting
I use Paypal and Stripe to process payments, neither one is working for 99% of customers when the issue happens
I have an upsell program called Selly but it seems to be working fine
I'm also using Omega multi pixels for my facebook pixels
I have google tags and use Google Analytics 4
Maybe if all the people who are reporting it on downdetector come here instead and talk in the comments Shopify will acknowledge this, or at least we can find a workaround. Telling all your customers to clear the cache is insane and obviously not a solution, anyone recommending that should just be fired on the spot honestly, but that's what support will tell you. Personally due to checkout issue's I've lost probably $20,000 already this month, I'm not sure how much longer I'll stick with this platform if it destroys my business like this. To save time, if shopify support shows up and requests screenshots (the chat support never knows anything and just reads from their script) - there are no screenshots of something not working for the customers to show, please just forward this info to someone on the tech side who could actually figure out what's going on. Thanks!