I've set up Paypal Express Checkout and Paypal Credit payments using my developer credentials. Regular sandbox Paypal payments go through and I can see the transactions in my sandbox user's account. When I try to pay with Paypal Credit, the login goes in an endless loop. These are the steps I take.
1. Select Paypal Credit from Magento checkout
2. Get redirected to Paypal's wth the following options
a. "Log in to Paypal to use Paypal Credit",
b. "Use Paypal Credit and create a Paypal account",
c. Debit/Credit card
3. Go to "Review your information" and click "Pay Now". Options a and b leads to the same pages eventually.
4. Go to a login screen again.
5. Go to "Review your information" again. Click "Pay Now". Repeat.