Intel AMT KVM connection issue

Hi folks

We try to use the integrated Intel AMT KVM functionality in RDM 10.5.3.0 (Rightclick on RDP Session of the POS Device > Intel AMT > Open KVM) but can't get a connection. First the "VNC Viewer Status for 127.0.0.1" window appears and after a while a VncViewer Message Box shows "Connection failed - End of Stream".

The Intel AMT KVM settings in the RDP session are: Display mode Embedded, VNC application UltraVNC, Redirection Intel AMT Redirection/TCP (16994)

Same happens when we create a dedicated Intel AMT KVM Session with the same settings (instead of an RDP session object).

Further info:
In RDM under File > Options > Tools > Intel AMT we set the same settings (display, vnc app and redirection)
Under File > Options > Path we configured VNC > UltraVNC to point to UltraVNC 1.2.0.5 ("Application is ready")
Other functions as power up, restart etc. are working fine

As a workaround we have installed VNC Viewer Plus and created a RDM "Tool" called "Intel(R) vPro(tm) VNC Viewer Plus" using "C:\Program Files (x86)\RealVNC\VNCViewerPlus\vncviewerplus.exe" with the uri parameters. This works. (Same credentials, same target port etc.)

Now my questions, as we'd like to use the integrated features in RDM
1. What are we doing wrong?
2. It is possible to choose VNC Viewer Plus instead of Ultra VNC as the integrated tool?

Thanks in advance for your help!

Regards
Daniel