The root problem is that Excel believes that there is data in row 65536
and/or column IV, so that inserting rows or columns would move data off the
grid.

To stop this issue, highlight A1 and press Ctrl+End. Note where your cursor
ends up (we are presuming Z65000 for this demo, so adjust to where you end
up). now you look to see where your 'actual' data finishes. (again, we will
assume G256 for this demo).

Select rows 65000 to 257 (that is entire rows, not just the cells) and
right-click a row number that is selected and select 'delete'. Not
'clear'...DELETE. Now select columns H to Z and do the same for columns as
you did for rows. Remember DELETE, not clear. Now save the workbook. (Very
important).

Now when you press Ctrl+End you should end up at G256 in our example and the
problem will have gone.