5a693e87_Plus__known_issues____RealVNC___Workaround
[ID] 128 [Description] Connecting users might not be authenticated automatically in a Kerberos network environment if an IP address was specified. [Status] Under consideration. [Workaround] 
Configure VNC Viewer Plus to look up a FQDN for the host computer.
VNC® Viewer Plus (known issues) - RealVNC®
Workaround