Error 10400 PayPal Express Checkout, please help.

This sort of thing typically happens if your session is getting wiped out after returning from PayPal during checkout. This can happen if you’re using a caching / CDN service on the web server or in WordPress directly.

I would try disabling such services and see if the problem goes away. If so, then we’ve found the culprit, and then it’s just a matter of configuring the service to work with your shopping cart, which typically means excluding cart/checkout pages from being included in the caching.

Please look into that and let me know what you find.

I have not heard back from you on this, so I’m going to go ahead and close this thread. Let me know if you’re still having trouble, though.