Some formulas are too difficult for Excel to evaluate without opening
the source workbook. (SUMIF is often cited).

One solution that sometimes works is to open the sources so that all
formulas give correct results then save the workbook and exit. On
re-opening the workbook (without the source workbooks being open,
errors will sometimes disappear).

The reason would be that it was last saved by a previous version of
Excel; on opening again in the later version a full recalculation is
performed and some formulas are then found to be too hard to compute
without fully opening the source workbooks.

Bill Manville
MVP - Microsoft Excel, Oxford, England
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