Overview

You notice that the Umbrella Connector service is displaying an error status on the Dashboard or does not appear in the dashboard and when looking in the Connector logs you see these errors:

1/30/2018 8:39:44 PM:Register() fail: The underlying connection was closed: An unexpected error occurred on a receive.

Explanation

This error is similar to the following. Please validate that https://support.umbrella.com/hc/en-us/articles/230902528-Umbrella-Connector-error-The-underlying-connection-was-closed-Could-not-establish-trust-relationship is not also being seen and start there. 

There are several possible causes for .NET to fail to reach the Umbrella API server even when the network is wide open. These causes are due to the .NET HTTP call via the .NET crypto API v2. To resolve this issue, follow these steps:

1. Uninstall .NET 3.5 if possible, and update the .NET 4.5 branch to .NET 4.7.x or higher. There are known issues with the minimum version to support TLS 1.2 4.6.2 which are resolved in .NET 4.7. 
   1. If uninstalling .NET 3.5 is not possible, see these steps to make it work with TLS 1.2 natively. https://support.umbrella.com/hc/en-us/articles/115005871543-Requirements-for-forcing-TLS-1-2-on-the-Connector-and-Roaming-Client 
2. Ensure that TLS 1.2 is not disabled, or that any erroneous registry keys are written to the registry's SCHANNEL TLS settings at HKLM SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols keys. Ensure that TLS 1.2 is not set to disabled, and there are no broken keys. Before making any changes to the registry, take a backup and refer to Microsoft's official instructions here. 
   
3. Reboot. After making .NET upgrades or registry key changes, reboot your machine.