Citrix VDA unregistered after removing/re adding into Domain.
It errors out as below "The Delivery controller failed to complete an audit of the sessions running on the VDA"

When we try to power mange VM's from studio we also receive Machine not found error.
 

Solution is same for both. These are MCS created machines so once the VM is created, the SID value of this is added into Citrix Database.
If we try to remove and re add this machine from domain, a new SID would be created but this is not updated in Citrix database.
Overriding DB tables to match new SID is not possible and supported as well hence either customer has to add a new Vm to existing catalog(remove older VM which is broken out of delivery group and MCS provisioned catalog) or if he wants to save the same machine. It can be done only as a manual provisioned VM by creating new manual catalog. 

Below snapshot shows the diff of 2 SID's. You can cross verify by using Get-brokermachine powershell command for value of SID in DB and cross verify its SID to AD.

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Problem Cause

Mismatch of SID's on Citrix database with Active directory 

Citrix VDA unregistered after removing/re adding into Domain. It errors out as below "The Delivery controller failed to complete an audit of the sessions running on the VDA"