Solution : https://service.sap.com/sap/support/notes/648372 (SAP Service marketplace login required)
Key words : 
normal workflow system function, edit wfts object type, method -> 'result type' tab, wfts bor object type, bor syntax check, valid data type, wfts object type, swh_str001         reference field, dateflds         reference field, data type reference
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