I noticed that this code also works for error 5992 when accessing the column width when the table has mixed column width.
Now to modify the width of a table (with vertically merged cells and cells with mixed width), I don't need to unmerge cells (and many ther steps) anymore, something simple like aCell.Width = 1.1 * aCell.Width do the trick.
Thank you very much.

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Backup your original file before doing any modification.