PayPal Security header is not valid
I ma getting the common "Security header is not valid" 10002 error code when trying to checkout with Paypal Payments Pro. This is a new site that is about to launch. I believe I have read every thread on the internet about this, and have gone through every "fix" and nothing has worked.
1. I am using live API credentials. I don't have a sandbox account so I am not using sandbox credentials. Also, eCart (6.08) is NOT toggled to "sandbox."
2. I have recreated my API's with PayPal, 4 times so this is not just a bad API. And, each time when I created the API's, I clicked that I accepted the agreement.
3. I cut and paste the API username, API password, and API Signature, and there are absolutely no leading or trailing spaces or missing characters.
4. I am using the correct credentials. I found a method for verifying with PayPal if the credentials are valid by populating a URL with the credentials. They are indeed valid and I am using the API credentials and not my PayPal account credentials.
5. I have recreated the ecart files using the checkout wizard twice, just to be on the safe side.
6. I am using SSL just in case is matters.
I have done everything I can thing of and have been up all night trying to figure this out. I am at my whits end. What could possibly be going wrong? I would really appreciate your help.