Solution : https://service.sap.com/sap/support/notes/1084953 (SAP Service marketplace login required)
Key words : 
support package 4sap netweaver 2004s, 19466  downloading sap kernel patches664679 installing 6, 125sap web application server, 200sap web application server, ascii-based systems, ebcdic-based systems, rfc server returns, rfc calls occur, rfc handshake mechanism, -converted communication codepage
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| 1319517 | Unicode Collection Note | 
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| 664679 | Installing 6.40 kernel in SAP WEB AS 6.10/6.20 | 
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| 19466 | Downloading SAP kernel patches |