The only solution I've found to this is to open the file on a Windows machine and resave it. The version/format is not important - it just has to be opened and resaved from a Windows box - then it will open properly under Office 98 for OS 9.
Other suggestions that don't involve going through Windows are very welcome.
[Editor's note: I haven't booted in OS 9 since early in 2001 so I can't easily test this hint myself (anyone else?). However, running Excel98 in Classic under 10.2, I had no problems opening any Excel v.X spreadsheets that I tested.]

