This is default behavior since NPrinting certificates that come with NPrinting server are not applicable to each unique environment.
The certificate warning can be ignored as this default certificate behavior has no impact to NPrinting normal operations
Optional: Secure the NPrinting Webconsole
You may choose to secure the NPrinting Web Console with a correctly configured 3rd party certificate. (Ensure that all servers and computers connecting to the NP server have the same trusted root certificate installed).
You may follow the self service related links in the section directly below or contact Qlik Professional services through your Qlik Account manager to assist with configuring third party certificates and securing the NPrinting Web Console.