This perfectly describes the problem I had with the ppa:x2go/stable version, both client and server on ubuntu 1604.
The crash of nxagent seems to be triggered by maximisation of the client window, e.g. when it is configured to use a whole display. If a static window size is configured, the crash happens when I move the client window. Maybe the unity window manager is sending some windowing events?
My workaround was to install from the ppa:x2go/saimaa repository.