This happened to few of my clients time to time in SharePoint online environments. You may have seen it but weirdly for some users only? You are not alone.
One of the errors is ‘Sorry we couldn’t open https://mantoso.sharepoint.com/DocumentLibrary/excelfile.xlsx’
And, the other error is – ‘Microsoft Excel cannot access the file ‘https://mantoso.sharepoint.com/DocumentLibrary/excelfile.xlsx’. There are several possible reasons:
- The file name or path does not exist.
- The file is being used by another program.
- The workbook you are trying to save has the same name as a currently open workbook.’
This issue in my perspective, can be caused by Office Document Cache of your Office Desktop application. Here’s how I managed to get rid of it.
Open Windows Explorer and copy and paste one of the following locations into the address bar:
Clearing Office Document Cache for Office 2016
%localappdata%\Microsoft\Office\16.0\OfficeFileCache
Clearing Office Document Cache for Office 2013
%localappdata%\Microsoft\Office\15.0\OfficeFileCache
Select all files beginning with ‘FS’
And delete those files. Restart the Excel application and you should now be able to open files from SharePoint.
Microsoft Cloud Infrastructure Technical Journal 
Thank you!
It was helpful.
Thanks for this one. Can confirm one more case where this worked – users stopped being able to access a file via https. Clearing out the cache files solved it. My user is happier now. Only challenge was getting past the protections we have, stopping people from messing around in LocalAppData to begin with.