I'm having this issue as well in Safari 11.0.3. When I test my paypal purchase buttons and links in Safari, I test them not logged into my paypal account, as most customers visiting my site won't initially be logged in. When I click the paypal button or link, Safari leaves my site and starts to load paypal,—I see PayPal, Inc. as the name of the browser tab— but the page stays white and never finishes loading.
As Mosabon suggested, if I login to paypal in a separate tab and then click the paypal button on my site, it opens the link successfully, but customers will not know to do this, and if I tell them, they may decide that it's too much trouble.
I don't think this is an issue with all types of paypal buttons/links, only the types that require the customer to have a paypal account—in my case, a Subscription button/link. But Safari is the only browser that seems to have this issue at all. I've tested my paypal subscription buttons/links in Chrome and Firefox, and they work appropriately—the paypal purchase page opens and the customer can sign into paypal (or create a paypal account) there if they choose to purchase.
Apple, please advise on a fix. This is a huge issue for anyone that processes payments via paypal.
Thanks so much!