Identified that the CITRIX XTE SERVER service terminated unexpectedly.

Solution/workaround

• Renamed the httpd.conf file in this directory C:\Program Files (x86)\Citrix\XTE\conf
• Copied the httpd.conf file from a working server
• Restarted the Citrix Xte Server Service

New sessions and existing sessions can now be established 

Current investigating the cause of the issue to ensure it will not happen again