The 404 means "file not found", which appears when the requested file (in the specified directory) is not present on the server.
That's normal because you can't install "both" Standard and Express at the same time, because that is redundant.
The "10002 Security error - Security header is not valid" error means you've not set your 3 PayPal credentials (sig, user, pwd) into the Express Checkout module, or the ones you've put in are for a different account or for a sandbox/test account instead of the Live/Production account.
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