I forgot to include the X2Go version details in my original bug report:

Server machine:
    * X2Go Server, Extensions and XSession version 4.0.1.19, from packages "x2goserver", "x2goserver-extensions" and "x2goserver-xsession" (4.0.1.19-0~1064~ubuntu14.04.1)
    * X2Go Desktop Sharing version 3.1.1.2, from package "x2godesktopsharing" (3.1.1.2-0~170~ubuntu14.04.1)

Client machine:
    * X2Go Client version 4.0.5.0, from package "x2goclient" (4.0.5.0-0~1099~ubuntu14.04.1)