Solution : https://service.sap.com/sap/support/notes/1174793 (SAP Service marketplace login required)
Key words :
terms cx_st_match_element sus xi purchaseorderchangerequest reason, /xi/sapglobal/global purchaseorderchangerequest, cx_st_match_element system expected, root-tag-q-, latest mapping file, xml message, element http, inbound checks, introduced additionally, prerequisites