We have also seen issues with the latest XenDesktop 7.x version where, sometimes, ListOfDDCs containing the FQDN of the DDC servers are not enough for successful VDA registrations.
In older versions, we used to configure a DNS alias for multiple Controllers to provide a DNS round robin load distribution among multiple servers. This doesn't work anymore with XenDesktop 7.x versions. So, it is recommended to use FQDN for all the Controller servers listed in the ListOfDDCs.
Sometimes, you have multiple Active Directory domains connected by an external trust and you want those VDAs from multiple domains to register to the Controller servers. In this scenario, it is necessary to create a registry key under the following relevant paths:
HKLM\Software\Citrix\VirtualDesktopAgent\HKLM\Software\Wow6432Node\Citrix\VirtualDesktopAgent\
You need to create a new string ListOfSIDs and set the value of the string to the SIDs of the Controllers (space delimited) similar to ListOfDDCs...