I tried to checkout a couple of days ago and got the following message:

We are sorry for the inconvenience. The PayPal account authentication settings are not yet set up, or the API security information is incorrect. We are unable to complete your transaction. Please notify the store owner so they can correct this problem. (10002)
Today, it seems to be working fine. I had the same looping issue when I used the host shared SSL. Purchasing a dedicated ssl fixed the issue.