XenDesktop Session Launches Then Disappears
You can update the AutoLogonTimeout registry key, however if your logins are taking longer than 90 seconds you should dig into why logins are so slow. XenDesktop and XenApp logins should on average take around 30 seconds from launch to being able to work within the desktop or application.
If your logins are are slow, start by taking a look at the Citrix Director and see where slow point is happening in the login process. Director will give you a good starting place.
Type : DWORD
90 is default. If your modifying this value it should be higher than 90.
Try logging in through the console to get an estimated time it takes to login then modify the value accordingly.