VNC viewer shows black screen without VPN access only

I access to a Siemens HMI screen which natively includes a VNC Server.

I get a black screen when connecting either with TightVNC or VNC Viewer. After a few seconds, the VNC connection ends up and the viewer stops.

Surprisingly, if I connect through a VPN access from a public network (Forticlient VPN), the display is correct and I don't get any black screen at all.

I have tried to modify several parameters from the VNC client, such as encoding type, colors, display size, etc. which usually help to resolve black screen issues, but nothing helped.

Any idea on potential causes and things I could test?

Thanks!

Thomas