3717020 - unable to add documents in purchase order header tab

In ME21N transaction an error is raised while inserting a Document Info Record (DIR) via the 'Linked Documents' functionality: 
"Document type & does not allow you to assign to object EKKO_PO" - Message no. 26050

• SAP Document Management System (CA-DMS)
• SAP S/4HANA on-premise

Purchase Order header, Document management, EKKO_PO, Document type error, TDWO table, SE16, SAP S/4HANA, Manage purchase orders, Harmonized Document Management, DMS, Header attachments, SAP Fiori, Error message, 26 050, DC10 , KBA , CA-DMS , Document management , PLM-FIO-DMS , Fiori UI for Document management , Problem