I changed some of the settings yesterday and now when I use the Sum Function the result shows the text of the function or ###
Column is not wide enough to display result. Widen the column.
You may have also changed the display of formulas. From menu → Tools->Options
under LibreOffice Calc->View
in right panel under Display
remove check for Formulas
.
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is an indicator that the calculated number does not fit into the cell. Increase your column width and you’ll see the number.