Solution : https://service.sap.com/sap/support/notes/916928 (SAP Service marketplace login required)
Key words : 
method reset_fobutype staticvisibility publicparameter           i_formex - importing - type, rsfob_s_formula-formex            i_objvers - importing - type, cx_sy_ref_is_initial cl_rsar_formulas save reason, objects_objref_not_assigned runtime error occurs, 0b            import support package 31, 10 content            import support package 25, 50            import support package 17, 0            import support package 08, reset_fobu method, support package
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