Weird behavior of column in Excel

Hi

I am having a problem. I use the Excel Addin to import the budget. If I write a value in a cell (say the letter 'A'), then I go back to the same cell and try to overwrite it with the letter 'B'. After entering the 'B', the spreadsheet rewrites the 'A' in the same cell and somehow it won't change.

Any ideas?

Steve