Error: Certificate validation failed. Authentication failed because the remote party has closed the transport stream.

Veeam console reports one of the following errors:

Certificate validation failed. Unable to connect to the service provider.

Certificate validation failed. Authentication failed because the remote party has closed the transport stream.

Multiple solutions:

Run Windows updates on the machine where Veeam Backup & Replication or Veeam Agent is installed. Further details can be found here: https://support.microsoft.com/en-us/help/3061518/ms15-055-vulnerability-in-schannel-could-allow-information-disclosure.
Open the ports required.
Allow the Anycloud DRaaS for Veeam service through the adaptive security.

There can be multiple reason to the error:

An update to the .NET Framework introduces a security check and do not allow to establish a secure connection to Anycloud DRaaS for Veeam
A connection to the Anycloud DRaaS for Veeam service cannot be established on the port TCP 6180.
Firewall that have adaptive security features like “deep packet inspection”, “packet inspection” or SSL/TLS inspection. This introduces a man-in-the-middel scenario and can cause issues to the exchange of the certificates.