As I attempt to update a sum of numbers in a file. When I try the plus sign seems accepted, but any number I try does not seem to accepted.

I am in Windows 7.
I am in Excel.
I have used this file for many years, without this occurring.
I assume the" err. 520", is an error code?

You say you are in Excel; are you in fact using LibreOffice Calc spreadsheet? What is the formula you are using?
Err 520 is an internal syntax error, suggesting your formula may be invalid.