Upload failure for files from device (desktop)

Ah OK, so you have identified the problem.

Unfortunately this is not a Teams issue as such as Teams just makes use of SharePoint. So if you are having issues in SharePoint, it is not going to get any better within Teams.

So someone has made modifications to the default Documents library, which for the best Teams experience is probably not advisable.

What I would recommend for future Teams, is that if a customised SharePoint Doc library is required, then either surface a SharePoint site via a Tab and interact with it on the page or add in an additional Document Library but leave the files tab default Documents library as is.

Back to the current issue, I would be looking to the modifications that have been made.

Thanks

Henry