When I try converting XLS to XLSX files, they appear to save correctly. But when I open and modify them, they open in compatibility mode with file names like "X000013" instead of the original file name, and are read-only. Also, converted files don't show up on the jump list. I even tried copying the text of one such file to a Word document , clearing the copy, and copying from that to a fresh XLSX file. The problem persists.

I havent tried any of these solutions but the website is reputable.
Let me know how you get along.

https://superuser.com/questions/5241...rt-xls-to-xlsx

I've found the solution: Control Panel \ Default programs showed the default for XLSX as Microsoft Open XLM Converter, rather than Outlook 2016.



Once I corrected that, updated worksheets saved under the oriignal name rather than with an arbitrary numeric label, and XLSX files showed up on the Jump List. I presume everything else is normal, too.

Thank you for sharing your solution

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Thanks NewMaven