Please ensure the .xls file is not opened in other workbooks, add-ins, or apps. After coming to that conclusion, simply attempt to open the file again in Excel to see if the problem no longer exists.

If you see The file couldn't open in protected view error while opening Microsoft Excel, the fixes listed in this post will help you.

Microsoft has announced that it will start disabling external workbook links to blocked file types by default between October 2025 and July 2026.

File-security limits can curtail your ability to open or save Microsoft Excel files.

Microsoft doesn't recommend re-enabling refreshing external links to blocked file types: "Disabling these protections reduces security and should only be done with caution." ...