I am getting the below error for a callout with the external system soon after the after the expiry of self signed certificate in salesforce and it has been replaced with new one with the same name. Prior to the certificate expiry the callout was working fine. The error received is as follows: Web service callout failed: Unexpected element. Parser was expecting element 'http://schemas.xmlsoap.org/soap/envelope/:Envelope' but found 'http://www.w3.org/1999/xhtml:html'
Could you please help on how this issue can be debugged?