UltraVNC

You hit the bullseye!

I followed your steps and now U-VNC behaves as it is supposed to...

"You may have been running the viewer with dsmplugin on AND saved setting as default."
I've to admit that this was indeed the first thing I did when installing Ultra VNC recently!

Thanks a lot for your ...

Ultra VNC SC works fine when the VNC viewer is executed with encryption (MSRC4Plugin.dsm) enabled:
vncviewer.exe -dsmplugin MSRC4Plugin.dsm -listen 54321
Inbound requests w/o encryption are dropped, only encrypted requests make it to my desk. Perfect.
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Running ...