Hi @Keith Vollero ,
The first thing to check is to make sure the current account signed in the local Excel desktop app is the one who has the read/edit permission to access the SharePoint site where the workbook is saved. If not, add the related account or add the specific SharePoint service. Steps are in the referenced thread: 'Sorry, we couldn't open https //d.docs.live.net' - Microsoft Community and Sharepoint error "Sorry, we couldn't open - Microsoft Community.
If the issue persists even the account is correct in the local Excel app, kindly do the following tests to narrow down the issue and further investigate it.
1.If the issue is only happening on this specific workbook? Try to create some new workbooks in that Document Library/other sites and open it from browser and local app to see if the issue will reproduce. This helps to make sure the issue is not from the SharePoint side.
2.If other users will be able to open the workbook without any error.
3.Some other users resolved the error by rename the issue workbook, download the workbook first and then open it from local Excel app(if this can work, save the file again to SharePoint and then rewrite the issue file to see if error will persist), clean the cache folder.
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