Payment Processing - Constantly Loading Icon

We use the PayPal Pro payment on our store and users have started experiencing an issue when checking out in that the spinning page loading icon just animates constantly and doesn't go away.

See the screenshot below.

Looks like users can still enter their details and actually checkout, but it is quite disconcerting. Has anyone else had the same issue?

If anyone wants to see for themselves they can do so here: https://tinyurl.com/yxv2z9p4

https://pasteboard.co/IqEgPkj.png

I've checked the error logs and there are no issues relating to that in there.

check the javascript developer console for errors, make sure you have error reporting enabled in your php settings, check your webserver error logs (not the opencart error log)