You get the following error trying to add VMware vCenter 6.5 or higher to Veeam Virtualization Extensions UI:
Cannot connect to "vCenter_Name on port 443". The underlying connection was closed: An unexpected error occurred on a send. Authentication failed because the remote party has closed the transport stream.
This is expected behavior if TLS 1.0 and SSLv3 are disabled on the target VMware Server.
To force .Net assemblies to use more secure encryption follow steps described in the following VMware KB article.