Check that all of the Qlik Sense services services are running. If no, make sure to start up any QS services that are not running.
If the above does not help, restart the host operating system
In the NPrinting Connection interface in the NP web console under 'Connections', use the NPrinting service account as the connection identity.
The NP service account use to run the NP Engine service should also be a rootadmin on the QS server. This ensures access to the QVF. If this account works then the identity previously used does not have sufficient permission to act as a connection identity.