I respond to this thread as I have a similar issue and dont get it to work.
my formula:
=SUMPRODUCT(--(RES_Project.Category=B57);--(RES_Accure.At="End");(RES_End.Date=YEAR(E$55));(RES_End.Date=MONTH($E55));(RES_Attributed.Costs))
B57 = Text
RES_Project.Category (Text)
=OFFSET(Resources!$E$9;;;MATCH(REPT("Z";255);Resources!$E$9:$E$506))
RES_Accure.At (Text)
=OFFSET(Resources!$P$9;;;MATCH(REPT("Z";255);Resources!$P$9:$P$506))
RES_End.Date (Date)
=OFFSET(Resources!$G$9;;;;MATCH(9,99999999999999E+307;Resources!$G:$G)-ROW(Resources!$G$9)+1)
RES_Attributed.Costs (amount EUR)
=OFFSET(Resources!$O$9;;;;MATCH(9,99999999999999E+307;Resources!$O:$O)-ROW(Resources!$O$9)+1)
Thanks for your help.