Incorrect cell value. To recalculate, I need Ctrl-Alt-F9 (F9 doesn't work)

I have a 2003 Excel spreadsheet in which a single cell has calculated to an incorrect value. Not a rounding error or bad formula or anything like that.

If I press Ctrl-Alt-F9, the cell recalculates to the correct value.

Automatic calculation is ON.

I can also make it recalculate by changing the value in a precedent cell and then changing it back, or Edit=>Replace "=" with "="

I am keen to see a solution that makes this error NOT HAPPEN, rather than relying on a workaround. This is because the spreadsheet is used by many different users in many organisations, and is very important to them (it calculates final money entitlements for staff exiting their employment).

Does anyone know what might be going on? I have searched the Microsoft (and this) forum in vain.

Richard