While you're able to to install an agent on some machines without persistent disks such as Thin client and Citrix PVS, Various functions stop working.
This is typically seen by performance counters not working as an example.
Previous attempts to resolve
While you may have resolved the issue yourself by following KB's like this
Or by logging a support ticket with Kaseya the problem reoccurs in a short time.
For Kaseya to function reliably it needs to be able to write to the disk and registry and have those changes persist after a reboot. Unfortunately Thin clients and Citrix PVS will revert to a previous state constantly removing any changes that were made to them. This will leave the machines out of sync with the Kserver and many other functions will simply fail.
In the case of monitoring the resolution is to monitor the machines externally with Kaseya network monitoring or Traverse.