My guess is that your X server isn't configured to listen for TCP connections (seems to be the default on modern Linux distros), and the cron job is missing something (permissions? environment variable?) to allow it to connect over unix domain sockets. xhost + won't help in that case, you need to either re-enable the tcp listening on the X server (varies by distro, look for "-nolisten tcp" in the X11 startup args) or figure out how to get the client to use unix domain sockets successfully from the cron job.