Sumifs for several worksheets

I have 19 identical workbooks, and need to get information from those into a master workbook. I've used SumIfs in the master workbook, and the code works perfect as long as the source workbook is open.
However, when I close excel and open the master workbook again, all formulas give a #Value message. When I open the source workbook again, the formulas give the values again. This is of course not optimal.

I've got an connection in the source workbook that I cant find or break, so every time I open those workbooks, I get the Update message box. No idea if this has anything to do with my problems.

Any ideas?

Regards

Hansi

Oh my, I didn`t realize you could have more than one condition in SUMPRODUCT. Thanks, mate.