Hello Vherbert,
Perhaps you have a range of cells in or near row 1048576 or column XFD that are not empty. Examine these areas. If you are sure that cells near the bottom or right of your worksheet should be empty, then I would suggest highlighting all rows or column below or to the right of your data and deleting these cells. Be sure to unhide all hidden cells by pressing Ctrl + A twice and then right-clicking to Unhide cells.