I am repeating this error on a vanilla install.

Reverting to previous versions of paypalwpp.php doesn't seem to help.

has any one had further success?

My position is that I am using the same set of credentials successfully elsewhere and have confirmed that they are not entered incorrectly. In other words, the problem site is a development version of a live site. Credentials work on the live site and do not work on the development site. I have copied and pasted the settings from Paypal (multiple times) rather than copying settings between the databases.

With Payflow UK I am getting the invalid amount error - although the amount seems fine ...

Code:
Array
(
    [PAYMENTREQUEST_0_AMT] => 0.47
    [PAYMENTREQUEST_0_ITEMAMT] => 0.47
    [L_PAYMENTREQUEST_0_NUMBER0] => PC-UNTM
    [L_PAYMENTREQUEST_0_NAME0] => Unreal Tournament Linked [22] Version: Download: Windows - English
    [L_PAYMENTREQUEST_0_QTY0] => 1
    [L_PAYMENTREQUEST_0_AMT0] => 0.47
    [PAYMENTREQUEST_0_CURRENCYCODE] => GBP
    [LOCALECODE] => GB
    [GIFTMESSAGEENABLE] => 0
    [GIFTRECEIPTEENABLE] => 0
    [GIFTWRAPENABLE] => 0
    [GIFTWRAPNAME] => 
    [GIFTWRAPAMOUNT] => 0
    [BUYEREMAILOPTINENABLE] => 0
    [CUSTOMERSERVICENUMBER] => 
    [BRANDNAME] => ####
    [PAYMENTREQUEST_0_PAYMENTACTION] => Sale
    [ALLOWNOTE] => 1
    [LANDINGPAGE] => Billing
    [NOSHIPPING] => 1
    [EMAIL] => ####
    [PAYMENTREQUEST_0_SHIPTOPHONENUM] => 078032124124
    [ACTION] => S
    [TENDER] => P
    [TRXTYPE] => S
    [RETURNURL[60]] => ####
    [CANCELURL[81]] => ####
    [ec_cancel] => 1
    [PAGESTYLE] => Primary
    [REQUEST_ID] => 1570195250
    [USER] => ####
    [PWD] => ####
    [PARTNER] => PayPalUK
    [VENDOR] => ####
)


Response: 
Array
(
    [RESULT] => 4
    [RESPMSG] => Invalid amount
    [REQUEST_ID] => 1570195250
    [CURL_ERRORS] => 
)


SetExpressCheckout, Elapsed: 906ms -- Failed

RESULT=4&RESPMSG=Invalid amount&REQUEST_ID=1570195250&CURL_ERRORS=
Anybody have any thoughts ?