There are several configuration/setup problems that can result in an inability to use the Horizon Client successfully

Problem Solution
Login Problems If you cannot log in to the Horizon Client, verify that the version of the VMware Horizon Client you are using is compatible with VMware View 5.1 or higher.
Desktop Does Not Launch If the Desktop does not launch, verify that no other software in the environment is using port 443.
Unable to Connect to Desktop If you receive the error message “Unable to Connect to Desktop,” it means that the View Agent is not running. In the Windows Control Panel programs, verify that Horizon Agent and View Agent Direct Connect appear in the list of installed programs. If they do not, the installation did not complete properly and you will need to reinstall. If the View Agent software is installed, verify that the View Agent Service is running.
Desktop Disconnects If a Horizon Client session ends too quickly when idle, this means that Horizon Client Session Timeout settings are configured to allow only a very short idle period. You can configure the Horizon Client Session Timeout settings in the administration console.