Devtools Console shows this error:
SyntaxError: Unexpected token '<', "<!--
The"... is not valid JSON
(anonymous) @ global.js?v=43297779302865413741681583315:1661
Which is coming from a script in the <head> of the theme
<script async="" src="https://cdn.shopify.com/bb5837980w7bcadfe3p20a28281m8e5071b1m.js"></script>
Which should be findable immediately after the <script id="web-pixels-manager-setup"></script> block of code though it's probably being injected as part of the necessary content_for_header tag.
Suspect some app is messing with the native fetch() function behavior and doing a bad job of it.
Typically seen in apps that try to auto-add or edit products being added to a cart.
If you've installed any such apps disable/remove them and test again after clearing the browser cache or in a different device|browser.
If you need it debugged further then can contact me directly for services.
Please provide context: store url, theme name, post url(s) and any further detail.
Contact Info in signature.
Good Hunting.
