I have a simple spreadsheet for tracking an account balance.

Column X is withdrawals, Y is deposits and Z is balance. An entry in Z25
would be =Z24-X25+Y25

Since line 25 will contain either a withdrawal or a deposit, one of the
cells in X or Y of line 25 will be empty, and the contents of that empty
cell will (supposedly) be assumed to be zero.

The spreadsheet has been working well and returning the values anticipated
until recently, when the cell in Z has been displaying #VALUE.

I can get the correct result by entering a zero in the empty cell in column
X or Y. However, this is a new development, something not necessary before.
Obviously something has changed in the way the value of the empty cell is
being determined, but I am unable to determine what it is.

All three columns have been formatted as number with two decimal digits and
a comma thousands separator..