VDA showed unregistered in Citrix Studio and ctxuvi event ID 1005 can be seen every 5 minutes in the affected VDA.
System event ID 2004 can be found before ctxuiv event ID 1003. System event ID 2004 means Windows has a low virtual memory issue.
Meanwhile, the detail of ctxvui event ID 1003 include error code C000017 which means status_no_memory.
Reboot the vda and increase Virtual Memory (Page File) on the VMs.
VDA showed unregistered in Citrix Studio and ctxuvi event ID 1005 can be seen every 5 minutes in the affected VDA