Sometimes Excel chooses to include a single quotation mark at the beginning of the formula ( '= ) and when I type Enter, it says: "We found an error in the formula and tried to correct for: ... Do you accept this correction?". Then he puts the formula, only without space and it does not work. I try to say that the cell is not text and does not solve.
It sometimes works when I copy, paste in Notepad, copy and paste in Excel back. Very strange...