Description of the Error:

Users in Microsoft Teams chat are displayed as Unknown user.

This can affect multiple users, single users or simply the chat preview.

Known Bug: Microsoft

Microsoft is aware of this bug and it is being tracked.

Lead forumn article for this: https://techcommunity.microsoft.com/t5/microsoft-teams/unknown-user-being-displayed/m-p/37625

Firstly, close down Microsoft Teams

Go to C:\Users\<Your staffNumber>\AppData\Roaming\Microsoft\Teams

Then delete all the content from this folder when Teams is closed,

Then restart Teams.

Note: The AppData folder may be hidden, so when looking in your folders, go to View at the top bar, then tick the "Show Hidden folders" box.