Need assistance for a case I created on the MTS support site. PayPal Pro API connection error


I need assistance determining why I cannot connect from my eCommerce website via the PayPal Pro API when I have verified all of my credentials are correct. When I view the log file after attempting to process a valid credit card, the message is "Security Header not valid" so PayPal blocks the transaction. No one from the MTS support team seems to be interested in replying to the case I created. I have already spent 3 days troubleshooting and am about ready to not use PayPal as my credit card processor. Any help or advice will be much appreciated.

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