Excel found unreadable content in 'stock.chart.object.xlsm'
 

When I create a secondary axis [because I want to overlay a moving average on
the bars] and save the workbook the chart is unrecoverable when I try to
reload the workbook later. I get a message saying "Excel found unreadable
content in 'stock.chart.object.xlsm'. Do you want to recover the contents of
this workbook?" When I reply 'yes' the spreadsheet is displayed without the
chart. A 'no' reply loses the entire workbook. I have tested it enough in
various ways to feel confident that it is probably the secondary axis
addition that is the problem.
Any suggestions will be much appreciated. Someone suggested a 'service pack
update' might help - whatever that is?
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JCMJCM