The operator “ ” expects a number, date or duration, but cell IN 19/09/2023 contains a string

The operator “ ” expects a number, date or duration, but cell IN 19/09/2023 contains a string

Can anyone help? Because this is driving me nuts.

The above is the error message I get after I've copied and pasted a working formula into the next cell down.

The formula has worked flawlessly for 174 lines (rows). Now, suddenly, I'm getting this error.

I've checked back on this community, and there have been several instances of this happening to others, with mixed results - some have solved it, others haven't.

The formula is not exactly complicated - it's just taking a starting number in one cell and subtracting the number in one cell and adding the cell in another. A simple case of A-B+C= whatever.

I'm using it as a very simple accounting tool. If I leave a cell blank, that means the value is zero. I've made sure that the numerical columns have been formatted as a number with 2 decimal places. It appears that, inexplicably, Numbers now thinks the blank cell contains text. It does not - it does not contain anything, it is completely blank!

The workaround is for me to simply insert a zero into the blank cell, then the formula works. But why is that suddenly necessary, when it has happily worked with blank cells for 174 lines??

Any assistance/suggestions appreciated...

Thanks.