Excel file opens read-only in specific library only in local app

[Stephan_TDI]

When opening an Excel file in local app via SharePoint, the Excel file is always in read-only.

Let me explain the situation:

Only one user has this issue at a firm, other users in the same authorization group don't experience this issue.

Issue is only on a specific document library, other libraries in the "Team" function like they should.

Only happens when opening it in local app, opening it in the web has the expected result.

Issue also shows when the user creates an Excel file and uploads it to that library.

So far the authorization has been checked, no specific authorization has been set. The user is in the same group as people who don't experience it. This makes me think it has to do with the Excel settings, but the issue doesn't show on another library so I have no clue what it could be.

Does anyone have any suggestions?

If more information is needed, please ask and I'll do my best to provide it. 

EDIT: Just confirmed that on another laptop it works fine, so it must be something in Excel. Still I don't understand why it only happens in this specific library. 

[#2]

Is this user syncing this library? Might be related. But only other way I could see this happening is if this user has an older Excel not supporting coauthoring so it see's read only, but I'm going to assume that they can just click on edit button on the banner and continue fine? I do know auto save saves per file now so once you change it, going forward it could be off, which would open in read only, check to see if this is on as well when opening file.