there's a black block follow the mouse cursor

[oper1000]

Please see the photo up

[RudiDeVos]

Version UltraVNC ?
Version OS, looks like win7 running in a virtual PC
This is a special situation, provide all steps to repeat it.
What mouse settings are you using?

Please provide instructions on how to repeat it

[oper1000]

Machine A : Windows 10 22H2 ---> Real PC

Version UltraVNC in Machine A : 1.4.3.1

Machine B : Windows 10 1903 ---> Real PC

Version UltraVNC in Machine B : 1.4.3.1

Machine C : Windows 7 6.1 7601 with SP1 ---> Vitrual PC

Machine C is no install UltraVNC.

Machine C is a virtual PC installed in Machine B

Now:

Machine A Remote Control Machine B , it's Normal

Machine A Remote Control Machine B and then use Machine C , will be display the black block

[Neustradamus]

@RudiDeVos: Have you seen?

[RudiDeVos]

In short, you open A connection from machine A to machine B and on machine B you have a VMWare workstation.
You open the desktop of the workstation with the VMWare buildin viewer and controle this using the VNC viewer on machine A.

This is complex to debug, before setting up a wmware workstation te test.
Have you already tried th 2 cursor options.

Did you know that you can also connect direct to a VMWare workstation, it has a bjuildin VNC server.