I'm not sure of any issues. I know paypal has deprecated Advanced Checkout. It is still supporting it for existing customers, but it doesn't plan on any updates, so that would imply that nothing would change on their side.
Unfortunately the first step is to reproduce the issue. My guess is that it has something to do with an item in the cart. Maybe a special character in the name or description. It is much more likely that something changed on your site to cause the problem than something on the paypal system failing intermittently. This implies to me that there is an item in your store that fails.