Unable to load target schema(xml type) in Mapping

Issue:
After migration from AIS to AC, the client is unable to load the target XML schema in the mapping. The process returns a “400 Bad Request” error, even though the same schema loads correctly in AIS.

Root Cause:
Upon extracting and reviewing the XSD of the target XML schema, we found that it references a validation URL. When attempting to access this URL directly in a web browser, it appears to be blocked. As a result, the schema cannot be validated during loading in AC, leading to the 400 Bad Request error.

Incorrect code : <!DOCTYPE cXML SYSTEM "http://xml.cxml.org/schemas/cXML/1.2.044/InvoiceDetail.dtd">
<cXML payloadID="BH1PL09092021098" timestamp="2021-08-06T00:00:00-00:00" version="1.2.044" xml:lang="en-US">

We have replaced the above code with the following code.

<cXML xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:noNamespaceSchemaLocation="cXML_1.2.044.xsd"
     payloadID="BH1PL09092021098"
     timestamp="2021-08-06T00:00:00-00:00"
     version="1.2.044"
     xml:lang="en-US">

After creating the XML layout, we noticed that the InvoicePartner's Email was missing. We added it to the XML structure, recreated the schema, and it is now visible.