Might be a bit late, but I managed to solve this issue for Canada Post. PayPal essentially automates Canada Post's EST online tool. The point being that if something would result in an error on the tool, PayPal will return an error itself. The one thing that caused this error code for me was the customs information. When giving the 'description of contents' you have to keep the character count less than 45 (the max limit on the EST tool)
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