Files became invisible in Teams channels but are still visible in SharePoint.
This could be caused by the document library that associated with the affected “Files” tab in Teams being renamed to something else.
Below are the steps taken to rename the affected document library back to its original name which fixed the issue.
1. Accessed Teams web app via Internet Explorer.
2. Opened the document library by clicking the “Open in SharePoint” link via the affected Team.
3. Clicked “Return to classic SharePoint”.
4. Clicked “Library” tab.
5. Clicked “Open in Explorer”.
6. Renamed the “Shared Documents” folder back to its original folder name which can be obtained from the URL that is shown on the address bar where you initially experiencing the error message.