TLDR: If you are signed in as root in the terminal and trying to start virt-manager you might get this error. Change to the same user that is signed into the GUI and launch virt-manager and the application should launch again.
This error occurs because virt-manager requires an active graphical session, which is not available when you are logged in as root in the terminal.
By switching to the same user who is currently logged into the GUI, you can launch virt-manager within an active graphical session and avoid the "unable to init server" error. You can do this by logging out of the root account and logging back in as the GUI user, or by using the
su command to switch to the GUI user in the terminal before launching virt-manager.