0) if you have dual screen or more
even if you have only single screen attached to graphic card have 2 display port
use uvnc 1.0.9.5 all are signed or above if available.
1)ultravnc server 1.0.8.0 and above.

You have set the same password for full and view only access.
2)any ultravnc server version... admin properties

You have checked [v] "Disable viewers inputs"
3) Server 1.0.8.2 is running as application on a Vista or WIN7 and you have an elevated "system" window open. To access "system" windows winvnc
need to be running as service.
4) ultravnc viewer (vncviewer) any version.
--Display--
[ ] View only (inputs ignored)
ultra vncviewer
Delete saved settings... close vncviewer... open vncviewer
if exist ... options.vnc or any name like filename.vnc file used by ultra vncviewer
viewonly=0 <-- input allowed (default)
viewonly=1 <-- keyboard input disabled
last solution, if all above solutions failed.
5. On controlled computer, Uninstall old ultravnc.... reboot... delete ultravnc folder
install new ultravnc
part 1 to 3 Rudi solutions
part 4 and 5 redge