This is in an MVC2 project, so I\'m using C# in ASP. This is what I\'m sending to https://api-3t.sandbox.paypal.com/nvp:
VERSION = 65.0 SIGNATURE = AFcWxV21C7fd0
From what I've read around, this can be a sign of malformed data. In my case, I was sending the form as
multipart/form-data
instead of
application/x-www-form-urlencoded
I hadn't read anywhere in the docs about form encoding, but as soon as I changed it the PP server got a lot more friendly.