If you turn on Debug Log to File in the PayPal Express Checkout module, it will store results of attempted transactions to your /includes/modules/payment/paypal/logs folder ( if it's writable ... otherwise it uses your /cache folder )
When you get errors like "(Transaction refused because of an invalid argument. See additional error messages for details.)" or "(Invalid HTTP Header)", it would be very helpful to be able to see these logs.
There are often 3 files with similar filenames but with some random 5-char extensions on them to prevent snoopers from being able to find and grab these files for inappropriate means. Please share the contents of all similarly-named files for analysis. You may obfuscate some data for privacy purposes, but please change as little as possible.
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