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
Workaround: Clear Teams local Cache
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.