Venafi Trust Protection Platform 15.4 or earlier when communicating with a CA that requires TLS 1.1 or 1.2 (Eg. Geotrust Reseller).
When trying to communicate with the CA TPP produces the error message "Error: CA Communication Error: The Request was aborted: Could not create SSL/TLS secure channel." The error message could also be "The underlying connection was closed: An unexpected error occurred on a send."
The connection we attempt with .Net Framework does not attempt to communicate using TLS 1.1 or 1.2 and therefore the connection attempt fails.
The Microsoft Security Advisory listed here solved this issue through an update to the .Net Framework. https://technet.microsoft.com/en-us/library/security/2960358.aspx
If you are unable to install the security update there is also a registry key change available that is listed on the same page.
Both methods require a reboot although the installers do not state this.