Hi Daniel
The ManageEngine agent has remote control capability and it could be that when the VNC protocol is intialised the ManageEngine Agent not the UltraVNC agent is responding.
Was VNC working before you added the ManageEngine agents?
If so I would reccomend un-installing the ManageEngine agent on a few machines and then testing UltraVNC connectivity.
The ManageEngine agent has remote control capability and it could be that when the VNC protocol is intialised the ManageEngine Agent not the UltraVNC agent is responding.
If removing the ManageEngine agent restores UltraVNC connectivity then a work around is to set the UltraVNC to communicate on non-standard ports.