What should I do to rectify this issue? Thank you.
NB: For privacy reasons, I have changed the name to John Doe.
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UNKNOWN part is your computer hostname, for some reasons it changed itself.
You can change the hostname from the Terminal using the following commands :
sudo scutil --set ComputerName "newname" sudo scutil --set LocalHostName "newname" sudo scutil --set HostName "newname"
Then flush your DNS cache with
dscacheutil -flushcache and reboot your Mac.
You can also change it without using commands, in
System Preferences > Sharing > Computer Name.