Fixed: VMware Horizon UAG "Unrecognized Request Detected" Warning

Fixed: VMware Horizon UAG "Unrecognized Request Detected" Warning Horizon UAG: Solving the 'Unrecognized Request Detected' Mystery Published: May 5, 2026 | Category: VDI Infrastructure & Security Overview of the Symptom In a VMware Horizon environment, administrators often encounter a yellow warning on the Unified Access Gateway (UAG) stating: "Unrecognized request detected." Curiously, the Connection Server remains healthy (Green), and the event often vanishes from the Horizon Console after a few minutes. 1. Root Cause Analysis This alert is a result of the UAG’s built-in security filtering. It triggers when the UAG receives traffic that does not conform to the expected Horizon protocol or HTTP standards. External Bot Scanning: As an edge device, the UAG is frequently hit by automated scanners (e....
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Troubleshooting VMware Horizon Client vdpConnect_Failure Issue

 

Introduction

VMware Horizon Client is a widely used solution for virtual desktop infrastructure (VDI). However, users may encounter various connection issues, one of which is the vdpConnect_Failure error. This error often results in a black screen followed by a disconnection, preventing users from accessing their virtual desktop. In this article, we will explore the possible causes of this issue and provide step-by-step solutions to resolve it.

Common Causes of vdpConnect_Failure

1. RPC Server Unavailable

One of the most common reasons for vdpConnect_Failure is the unavailability of the Remote Procedure Call (RPC) server. This can occur due to:

  • The RPC service being stopped or disabled

  • Network issues preventing communication with the server

  • Firewall blocking required ports

2. Display Protocol Issues (Blast/PCoIP/RDP)

Horizon Client uses different display protocols such as Blast, PCoIP, or RDP. If there are misconfigurations or compatibility issues with the selected protocol, the connection may fail.

3. Network or Firewall Restrictions

Horizon Client requires specific ports to be open for successful communication. If firewalls or security policies block these ports, the connection will fail.

4. Corrupt or Outdated VMware Horizon Agent

An outdated or corrupt Horizon Agent on the virtual desktop can lead to connectivity failures. Ensuring that the agent is properly installed and up-to-date is essential.

5. Authentication and Security Issues

Mismatched authentication settings between the client and the server can prevent a successful connection. Multi-Factor Authentication (MFA), certificate issues, or group policy restrictions may contribute to this problem.

6. Insufficient Virtual Machine Resources

If the virtual desktop lacks sufficient CPU, RAM, or GPU resources, it may not be able to handle the connection request, leading to failures.

Step-by-Step Solutions

✅ 1. Check RPC Service Status

  1. Open Run (Win + R) and type services.msc, then press Enter.

  2. Locate Remote Procedure Call (RPC) in the list.

  3. Ensure the service is running and set to Automatic.

  4. Restart the service if necessary.

✅ 2. Verify Display Protocol Configuration

  1. Open VMware Horizon Client.

  2. Before connecting, click on the settings (gear icon) and check the default display protocol.

  3. Try switching between PCoIP, Blast, and RDP to see if the issue persists.

  4. If using Blast, ensure that the VMware Blast Service is running on the virtual desktop.

✅ 3. Check Firewall and Network Settings

Ensure the following ports are open on the client, virtual desktop, and connection server:

ProtocolPortDescription
TCP443HTTPS (Horizon Connection Server)
TCP4172PCoIP display protocol
UDP4172PCoIP display protocol
TCP8443VMware Blast (HTML5 Access)
TCP3389RDP (if applicable)
TCP135RPC communication

Run the following command to check if ports are blocked:

netstat -an | find "4172"

If necessary, create firewall rules to allow traffic through these ports.

✅ 4. Restart VMware Horizon Agent on Virtual Desktop

  1. Open Run (Win + R) and type services.msc, then press Enter.

  2. Locate VMware Horizon View Agent.

  3. Restart the service and retry the connection.

  4. If restarting does not help, consider reinstalling the Horizon Agent on the VM.

✅ 5. Check Virtual Machine (VM) Health

  1. Open vCenter and select the virtual desktop.

  2. Check CPU and RAM usage.

  3. Ensure that VMware Tools and the Horizon Agent are installed and running.

  4. If necessary, increase VM resources (CPU, RAM, GPU).

✅ 6. Review Log Files for Detailed Errors

Check the following logs for additional insights:

  • Horizon Agent logs: C:\ProgramData\VMware\VDM\logs\debug.log

  • VMware Client logs: C:\Users\<Username>\AppData\Local\VMware\VDM\logs

  • ESXi/Host logs: /var/log/vmkwarning.log, /var/log/hostd.log

Look for errors related to vdpConnect_Failure or RPC Server Unavailable and troubleshoot accordingly.

Conclusion

The vdpConnect_Failure error in VMware Horizon Client can be frustrating, but by systematically troubleshooting the RPC service, network settings, display protocols, firewall configurations, and VM health, you can resolve the issue efficiently. If the problem persists, reviewing the logs will provide deeper insights into the root cause.

By following the steps outlined in this guide, you should be able to restore connectivity and access your virtual desktop seamlessly. If you need further assistance, consider reaching out to VMware support or checking the latest VMware documentation for updates.

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