@DanQuinlog I just tried a simpler formula on a sheet I created and it works with either CSV or XLSX, so I don't think that is the issue. I think your issue might be with the formula itself. Can you try with just a simple formula like "=SUM(D2:D4)" and see what happens?
@ThomasSasnett When I do a more simple formula like =SUM(C1:C10) it works, but of course that isn't the formula I'm trying to apply. When I tried SUMIFS and SUMIF I got only errors. After testing and double checking my formula was put together right, I guess the answer is those just aren't supported. SUMPRODUCT is older so I figured it might be supported, and appears to be supported since I am getting a value not an error, but its not returning the right value.
Is there any documentation about what formula are or are not supported with Excel Connector?