Been trying to tackle this issue all day. Our website uses Magento and PayPal to process payments.
When a customer checks out, everything works fine until you choose PayPal to check out. Using the Chrome dev console, you can see that it redirects to the same page over and over.
Things I've tried:
- Searching all over the net to no avail.
- Disabling all plugins and themes to no avail. (Website only has like 3 plugins)
- Re-configuring PayPal payment method, as well as using a different method.
- Using Sandbox mode in PayPal returns an error rather than a permanent payment loop.
- Applied updates to the site in hopes of fixing the issue.
- Tried using a different PayPal account and credentials; same result.
I'm at a loss. Help please! 
EDIT - Delete, Magento support and PayPal support were at a loss; reinstalled entire site.
Bump.