Be patient and wait for each page to load all it's stuff. graphics too. then hit the 'Continue" button. It took me three and a half hours to figure out, but that combine with the slash hack and paypal did the trick.
I just added an forward slash every time I got an error on every page I got an error and it went through.
Hope this helps
This is the key!!!!
Tried for hours, and then I got through easy once I just started spamming forward slashes. I was impatient, so I didn't do it one at a time. I already had it in my cart, so I went straight to: https://na.merch.riotgames.com/en/checkout/cart/ (with a bunch of added slashes).