I reached out to PayPal tech support regarding this error and this is what they said:
Thank you for contacting PayPal Merchant Technical Support.
PayPal has recently updated our checkout to a more streamlined experience. While most integrations work with this new PayPal checkout automatically, some integrations have issues that cause the user experience to fail. Some of the most common reasons for failed validation checks are:
Passing currency symbols ("$", "£", etc.) or values other than integers (1, 2, 3, etc.) and decimal points (“.”) in the amount
field. For example, "1234.50" is correct, but "$1,234.50" isn't allowed.
Passing country code
values greater than two letters. For example, "FR" is correct, but "France" or "fr_FR" isn't allowed.
Passing more than 127 characters in the item_number
field. For example, don't use the item_number field for "Note to Buyer" information.
Passing more than 64 characters in the option fields (on0
Passing duplicate empty parameters.
without passing an address.
The most pressing thing, and what is causing the error, is that your request is sending address_override=1, but no address is being passed. The minimum requirement is to send the shipping country, however it is recommended to pass values for address1, city, state, and zip as well. Alternatively, you can remove the address_override parameter and let the customer enter the shipping details themselves on the PayPal site.
In addition, there are several parameters being passed which are not a part of the PayPal Payments Standard library. These are currently being discarded from the request, but could cause problems in a future update. The unrecognized parameters are: PDT, AutoReturn, IPN, and login_email.
You can review the PayPal Payments Standard HTML library here: https://developer.paypal.com/docs/classic/paypal-payments-standard/integration-guide/Appx_websitestandard_htmlvariables/
Also, there is a microsite which explains the recent changes in greater detail here: https://www.paypal-knowledge.com/infocenter/index?page=content&id=FAQ2067&expand=true&locale=en_US