I recently encountered this error when the Paypal Express module was configured to require a confirmed Paypal Address, but the shipping address chosen during checkout did not match the verified address chosen in Paypal.

There were no changes on the Paypal account (using a Paypal balance) and when the zen cart address was modified to match the verified Paypal Address, the transaction completed normally.

I imagine disabling the verified address requirement would work around this issue, but could result in purchases not eligible for Paypal Seller Protection.