The most common cause of a Citrix HDX engine error message in WebEx conferencing platform is when the WebEx conferencing platform is unable to successfully communicate with the Citrix HDX engine. Begin with making sure you only have one instance of Citrix and one instance of WebEx running. Next, check whether your Adobe Flash player is current and running. Check whether IE is in protected mode. If it is, switch over to unprotected mode only for the duration of the Citrix session. Try restarting the Citrix session. If you still get an error, try rebooting the computer, followed by rebooting the server.