Problem with #invalidrefs

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I have built a datasheet, which pulls data from a source sheet ("PD Database"). I added over a hundred formula to the datasheet, along the lines of:

=SUMIFS({PD database Range 5}, {PD Database Range 4}, "PL", {PD Database Range 3}, "No")

The formula were all working fine, and data was being pulled across into the datasheet fine. But suddenly every single formula is now saying "#invalidref".

As far as I can see, the structure of the source sheet has not been changed (no new columns, no column re-ordering).

I have re-entered the first few formula and they work again. But I don't want to re-enter all 100 formula if it is going to break again.

The problem surely lies in the source sheet, or does it?

Any help is appreciated.

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