Dear User,
Good Day!! Thank you for posting your concern in this Microsoft Community.
Per the description shared, I would like to summarize your concern i.e., one of the user who is having access the SharePoint Online site is having the error when trying to open the Excel files from the synced folder to the Windows File Explorer via OneDrive sync client.
If my understanding is correct, as you have already performed the troubleshooting steps on the problematic device with no success, I would like to know whether the user is able to one the problematic files in Excel app via Open the Excel> Open> Sites - Company name
If yes, please try unlink the OneDrive account and shared libraries on the OneDrive sync client and setup the account by choosing the new location, then make sure to select the shortest path than before and see whether you encounter any error.
If yes, please create a new user profile on the problematic device and see whether you encounter the same issue. Reference: Create a local user or administrator account in Windows - Microsoft Support
Thank you for your patience and understanding!!
Sincerely
Mia | Microsoft Moderator