Symptoms

VDI agent status is listed as Not Verified in Parallels Remote Application Server console, as well as the virtual machines themselves.
Listing of the virtual machines and/or templates does not work and opening the respective tabs might cause Console to slow down.

The C:\Programdata\Parallels\RASLogs\controller.log contains the following records:

[D 0B/00000011/T0950/P0108] 16-10-19 11:42:54 - Establishing connection with remote VDI Provider (TestVDI-SRV).
[D 0C/00000035/T0950/P0108] 16-10-19 11:42:54 - Connecting to remote VDI host.
[E 0C/00001008/T0950/P0108] 16-10-19 11:42:54 - WMIConnect(Connect to remote server (\\TestVDI-SRV\root\MSCluster) failed : Err : 0x800706ba))
[E 0C/00001008/T0950/P0108] 16-10-19 11:42:54 - HyperV error occurred: 0x8. (Generic error code = 0x00008000)
[E 0C/00000035/T0950/P0108] 16-10-19 11:42:54 - Failed to connect to remote VDI host.
[E 0B/0000000D/T0950/P0108] 16-10-19 11:42:54 - Failed to establish connection with VDI Provider - Error[32]

Cause

One of the possible causes of this behavior is the lost WMI communication between the VDI agent (often installed on the publishing agent) and the Microsoft-based VDI hosts.
Please note that WMI functionality is essential for the Remote Application Server installation utilizing Windows-based agents.

Resolution

One of the fastest way to confirm the presence of the WMI communication between the Publishing Agent and the Microsoft Hyper-V host is to launch the WBEMTEST utility on the server running the Publishing agent and/or VDI agent.

WBEMTEST is available on any Windows system. Go to Start menu of your Publishing Agent machine and type "WBEMTEST" into the search or run box.
Once the program is launched, please enter the VDI host server address and use the same credentials that are used for running the VDI in Remote Application Server environment.

i.e. for a Hyper-V Failover Cluster the path could be the following:

\\winserver.domain.local\root\MSCluster

Here is an example of the program interface layout:
 

Once the address of the VDI host server and the right credentials are specified, press "Connect button".

Immediately after pressing the button you'd either get a successful connection or one of the various errors that would help you troubleshoot further.

More in-depth review of this utility could be found in the Introduction to WBEMTEST article on the Microsoft.com.
Please note that Scope of Support for Parallels Commercial products does not cover the issues with RPC and WMI.