As a regular user in NAV 2009 R2, attempting to Send a BizTalk document resulted in the error message.

This message is for C/AL programmers: The call to member Send failed: The handle is invalid

But I could send if the user happened to be in the network Administrators group.

The solution turned out to be due to permissions to the folder

C:\Documents and Settings\All Users\Application Data\Microsoft\Crypto\RSA\MachineKeys

Once I added read permissions to the user group "Everyone" I could send from NAV to BizTalk.

Looks like this folder is created by NAV but permissions are not inherited. I say this because other folders above and at the same level as MachineKeys had Read permissions for "Everyone"

As far as I can tell. The COM object being run by the NST needs to encrypt the message before it sends to the Commerce Gateway Requestor Service. Is was the encryption that was failing during the send.