Excel not able to write to Documents Folder

Hello,

I was asked to help a family member who was having trouble writing files with Excel, and I have to admit their problem has me totally flummoxed; so, I was hoping for some help myself.

When they open any Excel file, it says it is Read Only. Looking at document info confirms this, but we can't turn it off. Trying a Save As on the file generates a 0-byte file in the Documents folder and we then get a message that the file already exists, and we can't overwrite it.

I set up a new folder and added it to their Documents library. This works perfectly with Excel being able to Read/Write any file in there.

I thought it might be user permissions in Documents, but Word and Notepad can Read/Write in there without issue. The permission settings look good, and the problem is limited to Excel.

The problem started when they downloaded a CSV file, so the first thought was it was a virus. They sent me a copy of the file which I checked in an isolated VM environment, and it appeared OK. Both of us ran virus scans that came out clean. Key differences between our environments are: that they are using Microsoft 365 and Norton antivirus, and I'm using Office 2016 and Avast antivirus. But both can out of scans clean.

They have done a full reinstall of Microsoft 365 but the problem with Excel persists.

If worse comes to worst, then there might be a full PC rebuild (ouch), or migrate everything to the new folder I set up and delete the old Documents folder.

Ideally, I would like to remedy the cause of this issue which is why I'm asking for help.