This error is generated if the soap encoding schema file is not uploaded onto the Web Application Firewall.
To fix this, please visit http://schemas.xmlsoap.org/soap/encoding/, and save the resulting page as "soapschema.xsd".
After saving the file, import this file on the Web Application Firewall (under "Websites > XML Validations > Import Schema/WSDL").
After this file is uploaded, the WSDL file should upload without error.
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