XenServer 6.x and 7.x have been end-of-life and are unsupported. Customers still using these versions will see host connections no longer functioning when used with Citrix DaaS and Citrix Virtual Apps and Desktops (Version 2411 & later).
Customers should upgrade their Xenserver to a supported version in order to receive continuous support. The latest supported hypervisors for Citrix Machine Creation Services (MCS) and Citrix Provisioning Services (PVS) are listed in CTX131239.
The legacy product matrix is available at https://www.citrix.com/support/product-lifecycle/legacy-product-matrix.html