to commandline .
does the xming installation works without problem
as a standalone server?
please post on the environments you use.
xming system and system NXclient is communicating.
ensure that xming is working as standalone without problem by running
an xapp like xeyes etc.
if it is runing ok , connect opennx with appropriate proxy settings.
suggested basic configuration:
additionalparameters:clipboard in 'xlaunch'